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.

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?

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."

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.

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.

Image source, AFP

Nishikori shows what he is made of as he holds onto his serve to edge back in front in the second set.

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."

Another of your Facebook questions to former British number one Tim Henman

Image source, AFP

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."

Image source, AFP

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.

Andrew Castle
BBC tennis commentator on BBC Two

"An interesting phase here. For the first time this week a challenge for Novak Djokovic."

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

Image source, AFP

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.

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.

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."

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.

Hang on just a minute. We may have a game after all. Break point for Nishikori.

Tim Henman
Former British number one on BBC Two

"Plan A is not working for Nishikori. Is there any plan B?"

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.

Image source, AFP

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.

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."

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.

Image source, AFP

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.