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.

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."

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.

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, external

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

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."

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.

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:

Image source, AFP

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.

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.

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."

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.

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."

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.

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.

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."

Image source, AFP

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.

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.

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., external

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."

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.

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."