So that is everything from the first semi-final. We will be back at 20:00 GMT, for the second semi-final between Roger Federer and Stan Wawrinka. We will see you then. Bye for now.
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Andrew Castle
BBC tennis commentator on BBC Two
ReutersCopyright: Reuters
Nishikori will end the year as world number five, having started it as number 17, aided by that appearance in the US Open final. It's been a sensational year for him.
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Some words from our winner Novak Djokovic: "Kei was playing great and this has been the best season of his life but he made some crucial double faults in the third set.
"Of course I will watch the semi-final [between Roger Federer and Stan Wawrinka]. I'll get some popcorn and enjoy it from my bed."
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Djokovic 6-1 3-6 6-0 Nishikori
Novak Djokovic may be a bit more tired than he would like. He tries to sign the television camera with a bright yellow pen, but instead just leaves a dot on the screen, before he gives up.
But, he has certainly made his mark on the tournament.
He will face the winner of the second semi-final between Roger Federer and Stan Wawrinka on Sunday.
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Andrew Castle
BBC tennis commentator on BBC Two
"What a sad way for it to end but it's Djokovic who secures his place in the final where he will try to make it three in a row."
Game, set and match
Djokovic 6-1 3-6 6-0 Nishikori
He needs something special now, does Nishikori. But he loses the opening two points off his own serve. A cross-court winner after a Djokovic volley brings a smile to his face to make it 30-30, but an even better cross-court forehand from Djokovic gives the Serbian his first match point.
It is all over. A double fault. ends it.
AFPCopyright: AFP
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Tim Henman
Former British number one on BBC Two
"It will be a big disappointment if Nishikori's last set of the year is a bagel. I'd like to see him hold serve here."
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Djokovic 6-1 3-6 5-0 Nishikori*
AFPCopyright: AFP
The head is down, the shoulders are slumped. There looks no way back for Nishikori. He is galloping around frantically, but it is not enough. The class of Djokovic has proved decisive. He needs one more game. Nishikori serving to stay in the match.
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Tim Henman
Former British number one on BBC Two
"He battled so well in the second set to stay in it but here he is 15 minutes later and 4-0 and two breaks down in the decider."
Break of serve - Djokovic
*Djokoivc 6-1 3-6 4-0 Nishikori
Nishikori certainly has not given up, but is struggling for consistency. A double fault, his second of the set, puts him under more pressure, although he responds with an ace. His next point? Another double fault.
Those two mistakes help Djokovic move to break point, which he takes full advantage of. Nishikori throws his racquet to the floor in frustration. The racquet has gone. The match may soon be gone as well.
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Djokovic 6-1 3-6 3-0 Nishikori*
Good sportsmanship from Djokovic in the third game. A spectacular backhand cross-court winner from Nishikori is applauded by Djokovic. He was unimpressed with the crowd cheering on his opponent earlier in the match, but that was nice to see.
But that backhand winner can not lead to a break. Djokovic is back on top and looks just too good. He is now 3-0 ahead. Surely a place in the final is going to be his now?
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Andrew Castle
BBC tennis commentator on BBC Two
"Djokovic's indoor streak stands at 30. Ivan Lendl once won 37 on the trot, while John McEnroe holds the record at 59."
Break of serve - Djokovic
*Djokovic 6-1 3-6 2-0 Nishikori
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Break point for Djokovic now after Nishikori gives it too much welly and sees a forehand go long. Another one is overhit, which could give Djokovic the break.
It is challenged by the Japanese, but the umpire's call was correct. The ball was out. Djokovic back in charge.
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Piers Newbery
BBC Sport at the O2 Arena in London
"Finally this week, some real, big match drama. Djokovic barely made a winner after being annoyed by the crowd and he might well have been even more upset had Nishikori taken one of those two break points at the start of the decider.
"The Japanese player is, somewhat surprisingly, statistically the most successful player in the open era in deciding sets, and leads the ATP this year with a 21-2 record. A real test for the world number one who has won 14 of 16."
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Tim Henman
Former British number one on BBC Two
"It was a great opportunity for Nishikori in that first service game. But now the momentum changes."
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Djokovic 6-1 3-6 1-0 Nishikori*
The first set only took 23 minutes and it looked routine for the world number one, but Djokovic is having to battle hard for every point.
Nishikori, off Djokovic's serve, takes the opening two points, but is then furious with himself when he drags an angled forehand out.
However, an off-balance Djokovic then throws one well off target and Nishikori has two break points. The first ends disappointingly for him, with the ball in the net. The second one follows suit, and we're back at deuce.
Djokovic does the rest. The final point sees Nishikori diving the wrong way, like a baffled goalkeeper who has been given the eyes by a skilful penalty taker.
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Djokovic 6-1 3-6 Nishikori
Some stats for you from the second set...
Set time: 37 minutes
Winners: Nishikori 10, Djokovic 3
Unforced errors: Nishikori 5, Djokovic 8.
The world number one has lost a set for the first time this week. How will he respond?
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Tim Henman
Former British number one on BBC Two
"Djokovic's concentration levels dropped off after he was broken for the first time. It's going to be important mentally how both players come out at the start of the second set. I'm expecting a big reaction from Djokovic."
Second set - Nishikori
Djokoivc 6-1 3-6 Nishikori
It is a great time to produce your first ace of the match. Nishikori then has two set points. He only needs one of them. Djokovic's return goes wide to the left of the court and Nishikori takes the set. This match looked all over half an hour ago. Now it is one set each.
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Tim Henman
Former British number one on BBC Two
"He's been crushing his forehand well and suddenly from a set and a break down, Nishikori is serving for the second set."
Break of serve - Nishikori
Djokovic 6-1 3-5 Nishikori*
Aggressive play from Nishikori in the eighth game of the second set. He may be hampered by the strapping on his wrist, but still has enough skill to send Djokovic scuttling across to try and retrieve an angled forehand in the corner.
We mentioned earlier that Djokovic regarded Nishikori as one of the best baseline players in the game, and he shows why with another blaster that has Djokovic on the back foot.
The return is weak and the Japanese player dances forward to slap it away for a spectacular winner. That gives him break point. He then shows his range of shots, with a drop shot bringing Djokoivc to the net, a lob sending him back to create the opportunity for the forehand winner that sees him serving for the second set.
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*Djokovic 6-1 3-4 Nishikori
Congratulations, Kei.
You have just become the first man this week to win four games in a set against Novak Djokovic. Nishikori's coach Michael Chang nods and applauds approvingly. The world number five (Nishikori, not Chang) is hanging in there.
He restores his lead at 4-3 in the second set. However, if his task is not tough enough against a great of the game, Nishikori has to have some treatment on a wrist injury, which needs strapping up by a physio. How will that affect his performance?
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Tim Henman
Former British number one on BBC Two
"You've got to be prepared to force yourself through the pain barrier [when you're playing against someone like Djokovic]. You have to stick in there. Endurance is going to be built up on the practice courts."
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Djokovic 6-1 3-3 Nishikori*
Nishikori broke Djokovic's serve in the last game served by the Serbian. At 30-30 he has a chance to do it again, but is unsuccessful. We are level again.
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*Djokovic 6-1 2-3 Nishikori
Fans in Scotland that are watching on BBC Two need to press the Red Button now. Unless you want to watch the SNP Conference, of course.
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*Djokovic 6-2 2-3 Nishikori
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Nishikori shows what he is made of as he holds onto his serve to edge back in front in the second set.
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Djokovic 6-1 2-2 Nishikori*
Piers Newbery
BBC Sport at the O2 Arena in London
"There is no denying that was a pretty massive cheer after Djokovic double-faulted to lose serve, so much so that I'm not sure everyone in the arena would have noticed his reaction.
"He hardly needs the support but Djokovic has been disappointed in the past by the level of backing he gets in some of the big matches against the likes of Federer or Nadal.
"It's not much fun having your errors cheered but as a man with plenty of Davis Cup experience, it's surprising to see him that bothered."
Tim Henman Q and A
Another of your Facebook questions to former British number one Tim Henman
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Arjun Jethra: "Should the World Tour Finals stay in London beyond 2015?"
TH: "I think they should. There's an option for an extra year in 2016 and when the event has been as successful as it has with 260,000 people coming through the gates every year, great sponsors, very convenient location and a great city, I think it's going to have to be a very impressive bid for the World Tour Finals to move away from London."
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Djokovic 6-1 2-2 Nishikori*
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Just like the incredible hulk, you don't want to make Novak Djokovic angry. Normal service is resumed at the O2 as he blasts his way through the fourth game to quickly peg it back to 2-2.
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Andrew Castle
BBC tennis commentator on BBC Two
"An interesting phase here. For the first time this week a challenge for Novak Djokovic."
Get involved
We asked you if you agreed with John McEnroe, who said he would include tie-breaks in the final set of Grand Slam matches, such as at Wimbledon. Let us know your thoughts on Twitter via #bbctennis
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Elliot Harris Nixon: Couldn't imagine winning any Grand Slam on a tie break. Fed/Nadal Fed/Roddick at Wimbledon were tense classics!
RKGH: No way! Mahut vs Isner, Borg vs McEnroe, Ivanisevic vs Rafter, Federer vs Nadal - All legendary fifth sets
SJ Chowdhury: NO. More exciting without it. We want the drama of the deciding set.
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*Djokovic 6-1 1-2 Nishikori
It is not often you see Novak Djokovic rattled, but that is how he looks at the moment. The Japanese fans get a rare opportunity to wave their home-made banners in support of Nishikori as their man holds his serve to take a lead in the second set.
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Tim Henman
Former British number one on BBC Two
"Right now Djokovic is in such a commanding position he does not want to be distracted by the crowd."
Break of serve - Nishikori
Djokovic 6-1 1-1 Nishikori*
We have a break of serve. Djokovic double faults to show he is human.
He has not been impressed with the crowd cheering every Nishikori point and sarcastically applauds the crowd after his second serve hits the net to end the second game. The crowd were behind the underdog already, they may be even more so now.
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*Djokovic 6-1 1-0 Nishikori
Hang on just a minute. We may have a game after all. Break point for Nishikori.
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Tim Henman
Former British number one on BBC Two
"Plan A is not working for Nishikori. Is there any plan B?"
Break of serve - Djokovic
*Djokovic 6-1 1-0 Nishikori
Djokovic moves 15-0 up on Nishikori's serve in the opening game of the second set, before the Japanese player finally wins his first point behind a second serve. The London crowd want to see more tennis, so are cheering every successful point from Nishikori.
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However, there is only one more point to cheer. Djokovic carries on where he left off. He breaks Nishikori's serve to lead 1-0 in the second set, after taking the first 6-1.
Djokovic's results so far this week....
Beat Marin Cilic 6-1 6-1, beat Stan Wawrinka 6-3 6-0, beat Tomas Berdych 6-2 6-2. Phenomenal.
"I think the 56 minutes has been the shortest match of the week. That first set was just 23 minutes so we could be in for a record here."
First set - Djokovic
Djokovic 6-1 Nishikori
Coach Boris Becker looks furious as Djokovic misses a volley with the court wide open at 40-0. But Djokovic you old tease, he wins the set with the next point. It only took 23 minutes. Djokovic looks in a hurry and this one may not last long. My fingers can't type fast enough.
Break of serve
*Djokovic 5-1 Nishikori
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It is already an uphill battle for Nishikori. Djokovic makes a rare unforced error in the sixth game, but it still does not matter. We go to deuce, but a powerful forehand, followed by Nishikori drilling into the bottom of the net sees Djokovic serving for the set.
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Piers Newbery
BBC Sport at the O2 Arena in London
"Idemo Nole!" Djokovic has his fair share of Serbian support as he moves a break up but red and white is the colour scheme of the day. Plenty of Swiss fans have obviously bought tickets for both semi-finals, and a lot of Japanese flags are fluttering among the 17,000 spectators in this first semi. There is also a neat row of festive types wearing Christmas hats in one corner.
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Tim Henman
Former British number one on BBC Two
"It is only early days, just 15 minutes and five games into the match, but the pattern is continuing - 77% first serves in and he's won 12/13 points on first serve. How do you have an impact against a player who plays like Djokovic does? You can have a plan but it is about executing it."
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Djokovic 4-1 *Nishikori
Four shots later and it is 4-1. That's how good Djokovic is. And so quickly the first set looks out of Nishikori's reach. Tricky times for the Japanese player.
Get involved
We asked you if you agreed with John McEnroe, who said he would include tie-breaks in the final set of Grand Slam matches, such as at Wimbledon. Let us know your thoughts on Twitter via #bbctennis
BBCCopyright: BBC
Aidan Williams: No. It is about the spectacle not how quick a match is. Isner - Mahut was stunning.
Jeremy Day: Unquestionably not. Isner - Mahut was one of the greatest sporting events I have ever witnessed.
Andreas Plastiras: Imagine players would prefer it. Just ask Isner what he thought of chances in next round after the Mahut epic
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Andrew Castle
BBC tennis commentator on BBC Two
"It's a fabulous point from Djokovic to win the game. The sort of point he loves to win. The Japanese player would want that final volley back."
Break of serve
*Djokovic 3-1 Nishikori
At 0-30 Nishikori is in trouble of losing his serve in only his second service game. After winning one point, a return then goes long as he faces double break point. His first serve is out, but Djokovic can only blast one into the net. One break point saved, but there is another one.
Again the first serve is no good, a high-quality rally ensues before Nishikori punches a volley at the net wide.
Tim Henman Q and A
Former British number one Tim Henman has been doing a Q&A on the BBC Sport Facebook page. Here's a selection of some of his answers:
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Lesley Taylor: "Is it time for Murray to bring in a big hitter like John McEnroe?"
TH: "No I don't think it is. I think Murray knows his game. I think he's lost a little bit of direction with his game style right now - I think he's got a little bit too passive - but it's still been a good year, having been a difficult year coming back from back surgery . I'm looking forward to him responding in 2015."
We will have more answers from Tim throughout the match.
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Djokovic 2-1 *Nishikori
Well that one didn't last long. And we have our first use of the television replays. A forehand from Djokovic is in by the narrowest of margins and the scores moves onto 2-1.
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Andrew Castle
BBC tennis commentator on BBC Two
"He's already won 10% of all the games Djokovic has lost this week. It's something of a triumph just to hold serve."
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*Djokovic 1-1 Nishikori
A bullet forehand from Nishikori, followed by a blistering serve that Djokovic cannot return helps the underdog take his first game of the match. A confidence-boosting start for the Japanese player. Can he put Djokovic under pressure today? Marin Cilic, Stan Wawrinka and Tomas Berdych have all failed to do that so far this week.
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Djokovic 1-0 *Nishikori
So where is the game going to be won and lost? Djokovic is not taking anything for granted, despite only losing nine games in three matches so far this week.
"I played Kei about two weeks ago in Paris and I'm aware of his quality," said Djokovic. "He has one of the best baseline games in tennis. He is a talented young player and he played in the US Open final. He is playing the best tennis of his life and it's not going to be easy."
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Tim Henman
Former British number one on BBC Two
It's important Nishikori is clear in his mind with his game plan. It was a good start by Djokovic so it's vital Nishikori gets a couple of service games under his belt because Djokovic is such a good front runner.
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Djokovic 1-0 *Nishikori
So Tim Henman says there is nothing to lose for Kei Nishikori, but he can't do anything to stop Djokovic holding his serve in the opening game to take an early lead. A very efficient business-like start for the world number one.
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Djokovic v Nishikori
Tim Henman
Former British number one on BBC Two
"Nishikori has had a massive breakthrough season but we can't look to his serve for aces so he has to get a high percentage of first serves in. What I like about his game is his high-risk strategy, hitting balls over the net at its highest point.
"He's got to play aggressively. Djokovic is playing the best tennis of his life though so Nishikori has got nothing to lose. He can go out there and really let it fly."
Television coverage
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We are now live on BBC Two and you can also watch the coverage by hitting the play button at the top of this page.
If you are watching on BBC Two Scotland please be aware that you will need to watch via the red button from 15:00 GMT, because of coverage of the SNP Conference. We wouldn't want you to miss out on any of the action.
Coming up later
The second semi-final is an all Swiss affair as world number two Roger Federer faces Davis Cup team-mate Stan Wawrinka. They will be playing in the Davis Cup final against France next week, but friendships will be put to one side later on. That match gets under way at 20:00 GMT.
Get involved
#bbctennis
John McEnroe yesterday said that one of the changes he would make to tennis would be to include tie-breaks in the final set of matches.
So, is this a good or bad idea? What are the positives and negatives of this? Would you want to see a fifth set ended by a tie-break at Wimbledon.
Send us your thoughts and we will include the best responses on this page throughout the day. Contact us on Twitter, using the hashtag #bbctennis or leave a message on the BBC Sport Facebook page.
"That was some ovation for the world number one. He's really winning fans all over the world."
What an event
It is another packed arena at the O2 in London. This tournament might only be in London for one more year, but what a success it has been. The music is blaring out as the players walk into the arena.
'Djokovic indoors is becoming mission impossible'
Piers Newbery
BBC Sport at the O2 Arena in London
"Kei Nishikori will take plenty of confidence from that shock US open win over Djokovic, but the scenery has changed considerably.
"As someone lucky enough to have been at Flushing Meadows that day, I can confirm that it was a hellishly humid afternoon even by New York standards.
"Fast-forward two months and we are under the O2 roof, where Djokovic has not lost since 2012, and he hasn't been beaten anywhere indoors in 30 matches. Like Nadal on clay, Djokovic indoors is becoming the tennis version of mission impossible."
Djokovic rules
BBCCopyright: BBC
Put a roof on a building and Novak Djokovic is unstoppable. Admittedly, he is not too bad on other surfaces as well. Thirty times opponents have tried to beat him indoors since October 2012, 30 times they have failed.
You have to go all the way back to 31 October, 2012 for a second-round match of the Paris Masters for the Serbian's last indoor loss. Sam Querrey won 0-6 7-6 (7-5) 6-4.
Interestingly, Djokovic walked into the arena in Paris wearing a Darth Vader mask. The Force was certainly not with him though.
AFPCopyright: AFP
We meet again
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The man trying to stop Djokovic today is Japan's Kei Nishikori.
The 24-year-old became the first Asian player to qualify for the ATP World Tour Finals after reaching the US Open final in September, beating Djokovic in the semi-finals before losing to Marin Cilic in the final.
Will Nishikori be jumping for joy today? Stay with us. We will soon find out.
Good afternoon
Hello, good afternoon and welcome to BBC Sport's live text commentary of the first semi-final from the ATP World Tour Finals.
Novak Djokovic was the winner at the O2 in London in 2012 and 2013. No one has won the season-ending competition in three successive years since Ivan Lendl's successes in 1985, 1986 and 1987.
Who can stop Djokovic?
He has already secured the trophy for ending 2014 on top of the world rankings, and he may soon be looking to find more space in a very large and very packed trophy cabinet.
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Novak Djokovic has not lost an indoor match for more than two years and he has been in stunning form in London so far this week as he cruised into the semi-finals of the ATP World Tour Finals.
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Six sets played, six sets won. Who can stop him from winning the title for a third successive year?
Live Reporting
Michael Emons
All times stated are UK
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Latest PostPost update
So that is everything from the first semi-final. We will be back at 20:00 GMT, for the second semi-final between Roger Federer and Stan Wawrinka. We will see you then. Bye for now.
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Andrew Castle
BBC tennis commentator on BBC Two
Nishikori will end the year as world number five, having started it as number 17, aided by that appearance in the US Open final. It's been a sensational year for him.
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Some words from our winner Novak Djokovic: "Kei was playing great and this has been the best season of his life but he made some crucial double faults in the third set.
"Of course I will watch the semi-final [between Roger Federer and Stan Wawrinka]. I'll get some popcorn and enjoy it from my bed."
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Djokovic 6-1 3-6 6-0 Nishikori
Novak Djokovic may be a bit more tired than he would like. He tries to sign the television camera with a bright yellow pen, but instead just leaves a dot on the screen, before he gives up.
But, he has certainly made his mark on the tournament.
He will face the winner of the second semi-final between Roger Federer and Stan Wawrinka on Sunday.
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Andrew Castle
BBC tennis commentator on BBC Two
"What a sad way for it to end but it's Djokovic who secures his place in the final where he will try to make it three in a row."
Game, set and match
Djokovic 6-1 3-6 6-0 Nishikori
He needs something special now, does Nishikori. But he loses the opening two points off his own serve. A cross-court winner after a Djokovic volley brings a smile to his face to make it 30-30, but an even better cross-court forehand from Djokovic gives the Serbian his first match point.
It is all over. A double fault. ends it.
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Tim Henman
Former British number one on BBC Two
"It will be a big disappointment if Nishikori's last set of the year is a bagel. I'd like to see him hold serve here."
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Djokovic 6-1 3-6 5-0 Nishikori*
The head is down, the shoulders are slumped. There looks no way back for Nishikori. He is galloping around frantically, but it is not enough. The class of Djokovic has proved decisive. He needs one more game. Nishikori serving to stay in the match.
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Tim Henman
Former British number one on BBC Two
"He battled so well in the second set to stay in it but here he is 15 minutes later and 4-0 and two breaks down in the decider."
Break of serve - Djokovic
*Djokoivc 6-1 3-6 4-0 Nishikori
Nishikori certainly has not given up, but is struggling for consistency. A double fault, his second of the set, puts him under more pressure, although he responds with an ace. His next point? Another double fault.
Those two mistakes help Djokovic move to break point, which he takes full advantage of. Nishikori throws his racquet to the floor in frustration. The racquet has gone. The match may soon be gone as well.
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Djokovic 6-1 3-6 3-0 Nishikori*
Good sportsmanship from Djokovic in the third game. A spectacular backhand cross-court winner from Nishikori is applauded by Djokovic. He was unimpressed with the crowd cheering on his opponent earlier in the match, but that was nice to see.
But that backhand winner can not lead to a break. Djokovic is back on top and looks just too good. He is now 3-0 ahead. Surely a place in the final is going to be his now?
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Andrew Castle
BBC tennis commentator on BBC Two
"Djokovic's indoor streak stands at 30. Ivan Lendl once won 37 on the trot, while John McEnroe holds the record at 59."
Break of serve - Djokovic
*Djokovic 6-1 3-6 2-0 Nishikori
Break point for Djokovic now after Nishikori gives it too much welly and sees a forehand go long. Another one is overhit, which could give Djokovic the break.
It is challenged by the Japanese, but the umpire's call was correct. The ball was out. Djokovic back in charge.
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Piers Newbery
BBC Sport at the O2 Arena in London
"Finally this week, some real, big match drama. Djokovic barely made a winner after being annoyed by the crowd and he might well have been even more upset had Nishikori taken one of those two break points at the start of the decider.
"The Japanese player is, somewhat surprisingly, statistically the most successful player in the open era in deciding sets, and leads the ATP this year with a 21-2 record. A real test for the world number one who has won 14 of 16."
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Tim Henman
Former British number one on BBC Two
"It was a great opportunity for Nishikori in that first service game. But now the momentum changes."
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Djokovic 6-1 3-6 1-0 Nishikori*
The first set only took 23 minutes and it looked routine for the world number one, but Djokovic is having to battle hard for every point.
Nishikori, off Djokovic's serve, takes the opening two points, but is then furious with himself when he drags an angled forehand out.
However, an off-balance Djokovic then throws one well off target and Nishikori has two break points. The first ends disappointingly for him, with the ball in the net. The second one follows suit, and we're back at deuce.
Djokovic does the rest. The final point sees Nishikori diving the wrong way, like a baffled goalkeeper who has been given the eyes by a skilful penalty taker.
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Djokovic 6-1 3-6 Nishikori
Some stats for you from the second set...
Set time: 37 minutes
Winners: Nishikori 10, Djokovic 3
Unforced errors: Nishikori 5, Djokovic 8.
The world number one has lost a set for the first time this week. How will he respond?
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Tim Henman
Former British number one on BBC Two
"Djokovic's concentration levels dropped off after he was broken for the first time. It's going to be important mentally how both players come out at the start of the second set. I'm expecting a big reaction from Djokovic."
Second set - Nishikori
Djokoivc 6-1 3-6 Nishikori
It is a great time to produce your first ace of the match. Nishikori then has two set points. He only needs one of them. Djokovic's return goes wide to the left of the court and Nishikori takes the set. This match looked all over half an hour ago. Now it is one set each.
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Tim Henman
Former British number one on BBC Two
"He's been crushing his forehand well and suddenly from a set and a break down, Nishikori is serving for the second set."
Break of serve - Nishikori
Djokovic 6-1 3-5 Nishikori*
Aggressive play from Nishikori in the eighth game of the second set. He may be hampered by the strapping on his wrist, but still has enough skill to send Djokovic scuttling across to try and retrieve an angled forehand in the corner.
We mentioned earlier that Djokovic regarded Nishikori as one of the best baseline players in the game, and he shows why with another blaster that has Djokovic on the back foot.
The return is weak and the Japanese player dances forward to slap it away for a spectacular winner. That gives him break point. He then shows his range of shots, with a drop shot bringing Djokoivc to the net, a lob sending him back to create the opportunity for the forehand winner that sees him serving for the second set.
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*Djokovic 6-1 3-4 Nishikori
Congratulations, Kei.
You have just become the first man this week to win four games in a set against Novak Djokovic. Nishikori's coach Michael Chang nods and applauds approvingly. The world number five (Nishikori, not Chang) is hanging in there.
He restores his lead at 4-3 in the second set. However, if his task is not tough enough against a great of the game, Nishikori has to have some treatment on a wrist injury, which needs strapping up by a physio. How will that affect his performance?
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Tim Henman
Former British number one on BBC Two
"You've got to be prepared to force yourself through the pain barrier [when you're playing against someone like Djokovic]. You have to stick in there. Endurance is going to be built up on the practice courts."
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Djokovic 6-1 3-3 Nishikori*
Nishikori broke Djokovic's serve in the last game served by the Serbian. At 30-30 he has a chance to do it again, but is unsuccessful. We are level again.
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*Djokovic 6-1 2-3 Nishikori
Fans in Scotland that are watching on BBC Two need to press the Red Button now. Unless you want to watch the SNP Conference, of course.
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*Djokovic 6-2 2-3 Nishikori
Nishikori shows what he is made of as he holds onto his serve to edge back in front in the second set.
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Djokovic 6-1 2-2 Nishikori*
Piers Newbery
BBC Sport at the O2 Arena in London
"There is no denying that was a pretty massive cheer after Djokovic double-faulted to lose serve, so much so that I'm not sure everyone in the arena would have noticed his reaction.
"He hardly needs the support but Djokovic has been disappointed in the past by the level of backing he gets in some of the big matches against the likes of Federer or Nadal.
"It's not much fun having your errors cheered but as a man with plenty of Davis Cup experience, it's surprising to see him that bothered."
Tim Henman Q and A
Another of your Facebook questions to former British number one Tim Henman
Arjun Jethra: "Should the World Tour Finals stay in London beyond 2015?"
TH: "I think they should. There's an option for an extra year in 2016 and when the event has been as successful as it has with 260,000 people coming through the gates every year, great sponsors, very convenient location and a great city, I think it's going to have to be a very impressive bid for the World Tour Finals to move away from London."
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Djokovic 6-1 2-2 Nishikori*
Just like the incredible hulk, you don't want to make Novak Djokovic angry. Normal service is resumed at the O2 as he blasts his way through the fourth game to quickly peg it back to 2-2.
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Andrew Castle
BBC tennis commentator on BBC Two
"An interesting phase here. For the first time this week a challenge for Novak Djokovic."
Get involved
We asked you if you agreed with John McEnroe, who said he would include tie-breaks in the final set of Grand Slam matches, such as at Wimbledon. Let us know your thoughts on Twitter via #bbctennis
Elliot Harris Nixon: Couldn't imagine winning any Grand Slam on a tie break. Fed/Nadal Fed/Roddick at Wimbledon were tense classics!
RKGH: No way! Mahut vs Isner, Borg vs McEnroe, Ivanisevic vs Rafter, Federer vs Nadal - All legendary fifth sets
SJ Chowdhury: NO. More exciting without it. We want the drama of the deciding set.
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*Djokovic 6-1 1-2 Nishikori
It is not often you see Novak Djokovic rattled, but that is how he looks at the moment. The Japanese fans get a rare opportunity to wave their home-made banners in support of Nishikori as their man holds his serve to take a lead in the second set.
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Tim Henman
Former British number one on BBC Two
"Right now Djokovic is in such a commanding position he does not want to be distracted by the crowd."
Break of serve - Nishikori
Djokovic 6-1 1-1 Nishikori*
We have a break of serve. Djokovic double faults to show he is human.
He has not been impressed with the crowd cheering every Nishikori point and sarcastically applauds the crowd after his second serve hits the net to end the second game. The crowd were behind the underdog already, they may be even more so now.
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*Djokovic 6-1 1-0 Nishikori
Hang on just a minute. We may have a game after all. Break point for Nishikori.
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Tim Henman
Former British number one on BBC Two
"Plan A is not working for Nishikori. Is there any plan B?"
Break of serve - Djokovic
*Djokovic 6-1 1-0 Nishikori
Djokovic moves 15-0 up on Nishikori's serve in the opening game of the second set, before the Japanese player finally wins his first point behind a second serve. The London crowd want to see more tennis, so are cheering every successful point from Nishikori.
However, there is only one more point to cheer. Djokovic carries on where he left off. He breaks Nishikori's serve to lead 1-0 in the second set, after taking the first 6-1.
Djokovic's results so far this week....
Beat Marin Cilic 6-1 6-1, beat Stan Wawrinka 6-3 6-0, beat Tomas Berdych 6-2 6-2. Phenomenal.
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Sue Barker
BBC tennis presenter on BBC Two
"I think the 56 minutes has been the shortest match of the week. That first set was just 23 minutes so we could be in for a record here."
First set - Djokovic
Djokovic 6-1 Nishikori
Coach Boris Becker looks furious as Djokovic misses a volley with the court wide open at 40-0. But Djokovic you old tease, he wins the set with the next point. It only took 23 minutes. Djokovic looks in a hurry and this one may not last long. My fingers can't type fast enough.
Break of serve
*Djokovic 5-1 Nishikori
It is already an uphill battle for Nishikori. Djokovic makes a rare unforced error in the sixth game, but it still does not matter. We go to deuce, but a powerful forehand, followed by Nishikori drilling into the bottom of the net sees Djokovic serving for the set.
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Piers Newbery
BBC Sport at the O2 Arena in London
"Idemo Nole!" Djokovic has his fair share of Serbian support as he moves a break up but red and white is the colour scheme of the day. Plenty of Swiss fans have obviously bought tickets for both semi-finals, and a lot of Japanese flags are fluttering among the 17,000 spectators in this first semi. There is also a neat row of festive types wearing Christmas hats in one corner.
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Tim Henman
Former British number one on BBC Two
"It is only early days, just 15 minutes and five games into the match, but the pattern is continuing - 77% first serves in and he's won 12/13 points on first serve. How do you have an impact against a player who plays like Djokovic does? You can have a plan but it is about executing it."
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Djokovic 4-1 *Nishikori
Four shots later and it is 4-1. That's how good Djokovic is. And so quickly the first set looks out of Nishikori's reach. Tricky times for the Japanese player.
Get involved
We asked you if you agreed with John McEnroe, who said he would include tie-breaks in the final set of Grand Slam matches, such as at Wimbledon. Let us know your thoughts on Twitter via #bbctennis
Aidan Williams: No. It is about the spectacle not how quick a match is. Isner - Mahut was stunning.
Jeremy Day: Unquestionably not. Isner - Mahut was one of the greatest sporting events I have ever witnessed.
Andreas Plastiras: Imagine players would prefer it. Just ask Isner what he thought of chances in next round after the Mahut epic
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Andrew Castle
BBC tennis commentator on BBC Two
"It's a fabulous point from Djokovic to win the game. The sort of point he loves to win. The Japanese player would want that final volley back."
Break of serve
*Djokovic 3-1 Nishikori
At 0-30 Nishikori is in trouble of losing his serve in only his second service game. After winning one point, a return then goes long as he faces double break point. His first serve is out, but Djokovic can only blast one into the net. One break point saved, but there is another one.
Again the first serve is no good, a high-quality rally ensues before Nishikori punches a volley at the net wide.
Tim Henman Q and A
Former British number one Tim Henman has been doing a Q&A on the BBC Sport Facebook page. Here's a selection of some of his answers:
Lesley Taylor: "Is it time for Murray to bring in a big hitter like John McEnroe?"
TH: "No I don't think it is. I think Murray knows his game. I think he's lost a little bit of direction with his game style right now - I think he's got a little bit too passive - but it's still been a good year, having been a difficult year coming back from back surgery . I'm looking forward to him responding in 2015."
We will have more answers from Tim throughout the match.
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Djokovic 2-1 *Nishikori
Well that one didn't last long. And we have our first use of the television replays. A forehand from Djokovic is in by the narrowest of margins and the scores moves onto 2-1.
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Andrew Castle
BBC tennis commentator on BBC Two
"He's already won 10% of all the games Djokovic has lost this week. It's something of a triumph just to hold serve."
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*Djokovic 1-1 Nishikori
A bullet forehand from Nishikori, followed by a blistering serve that Djokovic cannot return helps the underdog take his first game of the match. A confidence-boosting start for the Japanese player. Can he put Djokovic under pressure today? Marin Cilic, Stan Wawrinka and Tomas Berdych have all failed to do that so far this week.
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Djokovic 1-0 *Nishikori
So where is the game going to be won and lost? Djokovic is not taking anything for granted, despite only losing nine games in three matches so far this week.
"I played Kei about two weeks ago in Paris and I'm aware of his quality," said Djokovic. "He has one of the best baseline games in tennis. He is a talented young player and he played in the US Open final. He is playing the best tennis of his life and it's not going to be easy."
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Tim Henman
Former British number one on BBC Two
It's important Nishikori is clear in his mind with his game plan. It was a good start by Djokovic so it's vital Nishikori gets a couple of service games under his belt because Djokovic is such a good front runner.
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Djokovic 1-0 *Nishikori
So Tim Henman says there is nothing to lose for Kei Nishikori, but he can't do anything to stop Djokovic holding his serve in the opening game to take an early lead. A very efficient business-like start for the world number one.
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Djokovic v Nishikori
Tim Henman
Former British number one on BBC Two
"Nishikori has had a massive breakthrough season but we can't look to his serve for aces so he has to get a high percentage of first serves in. What I like about his game is his high-risk strategy, hitting balls over the net at its highest point.
"He's got to play aggressively. Djokovic is playing the best tennis of his life though so Nishikori has got nothing to lose. He can go out there and really let it fly."
Television coverage
We are now live on BBC Two and you can also watch the coverage by hitting the play button at the top of this page.
If you are watching on BBC Two Scotland please be aware that you will need to watch via the red button from 15:00 GMT, because of coverage of the SNP Conference. We wouldn't want you to miss out on any of the action.
Coming up later
The second semi-final is an all Swiss affair as world number two Roger Federer faces Davis Cup team-mate Stan Wawrinka. They will be playing in the Davis Cup final against France next week, but friendships will be put to one side later on. That match gets under way at 20:00 GMT.
Get involved
#bbctennis
John McEnroe yesterday said that one of the changes he would make to tennis would be to include tie-breaks in the final set of matches.
So, is this a good or bad idea? What are the positives and negatives of this? Would you want to see a fifth set ended by a tie-break at Wimbledon.
Send us your thoughts and we will include the best responses on this page throughout the day. Contact us on Twitter, using the hashtag #bbctennis or leave a message on the BBC Sport Facebook page.
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Sue Barker
BBC tennis presenter on BBC Two
"That was some ovation for the world number one. He's really winning fans all over the world."
What an event
It is another packed arena at the O2 in London. This tournament might only be in London for one more year, but what a success it has been. The music is blaring out as the players walk into the arena.
'Djokovic indoors is becoming mission impossible'
Piers Newbery
BBC Sport at the O2 Arena in London
"Kei Nishikori will take plenty of confidence from that shock US open win over Djokovic, but the scenery has changed considerably.
"As someone lucky enough to have been at Flushing Meadows that day, I can confirm that it was a hellishly humid afternoon even by New York standards.
"Fast-forward two months and we are under the O2 roof, where Djokovic has not lost since 2012, and he hasn't been beaten anywhere indoors in 30 matches. Like Nadal on clay, Djokovic indoors is becoming the tennis version of mission impossible."
Djokovic rules
Put a roof on a building and Novak Djokovic is unstoppable. Admittedly, he is not too bad on other surfaces as well. Thirty times opponents have tried to beat him indoors since October 2012, 30 times they have failed.
You have to go all the way back to 31 October, 2012 for a second-round match of the Paris Masters for the Serbian's last indoor loss. Sam Querrey won 0-6 7-6 (7-5) 6-4.
Interestingly, Djokovic walked into the arena in Paris wearing a Darth Vader mask. The Force was certainly not with him though.
We meet again
The man trying to stop Djokovic today is Japan's Kei Nishikori.
The 24-year-old became the first Asian player to qualify for the ATP World Tour Finals after reaching the US Open final in September, beating Djokovic in the semi-finals before losing to Marin Cilic in the final.
Will Nishikori be jumping for joy today? Stay with us. We will soon find out.
Good afternoon
Hello, good afternoon and welcome to BBC Sport's live text commentary of the first semi-final from the ATP World Tour Finals.
Novak Djokovic was the winner at the O2 in London in 2012 and 2013. No one has won the season-ending competition in three successive years since Ivan Lendl's successes in 1985, 1986 and 1987.
Who can stop Djokovic?
He has already secured the trophy for ending 2014 on top of the world rankings, and he may soon be looking to find more space in a very large and very packed trophy cabinet.
Novak Djokovic has not lost an indoor match for more than two years and he has been in stunning form in London so far this week as he cruised into the semi-finals of the ATP World Tour Finals.
Six sets played, six sets won. Who can stop him from winning the title for a third successive year?