Now it is time for Federer to impress with his serve. After missing a chance to finish the game off when, while positioned at the net, he deflects a volley wide, the world number two then smashes down a serve, which Wawrinka can't keep in play. The second set remains on serve.

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

"Stan will be a little bit thankful that he won that service game. He needs to get more first serves in, he needs to get his percentage up."

Wawrinka's serve was formidable in the opening set, but early in the second he is struggling to get his first serves in. Federer has a chance at 30-30, before he completely misjudges a backhand.

The ball bounces off the top of the racquet and flies high into the air. You won't see shots like that too many times from the 17-time Grand Slam winner. Wawrinka holds and we are level early in the second set.

After Djokovic lost a set earlier today to Kei Nishikori, he then won the next one 6-0. Federer breezes through his opening service game of the second set and looks very focused. Let's see how good Wawrinka is now.

More from Tim Henman's Facebook Q&A., external

Image source, AFP

Adam Street: Is Amelie Mauresmo the right coach to take Andy Murray's career forward? I can imagine that Lendl would have been more of an authoritative figure because the titles he has won and his personality. Is Mauresmo the right person to do the same?

TH: It's difficult to say as they haven't really worked together for a particularly long time. Let's not forget that Murray won three of his last six tournaments at the end of the year, so the only person who has the answer to that is Andy himself.

So, before the second set starts, let's have a look at how Andy Murray recovers from his disappointing defeat by Roger Federer earlier in the week. BBC Sport has spoken to John McEnroe, Tim Henman and Mats Wilander to find out.

Read the feature on the BBC Tennis page.

I wonder what Andy Murray makes of Roger Federer's performance today, after being thrashed by the Swiss on Thursday.

In case you have missed it, Murray's latest column on the BBC Sport website is well worth a read as he discusses his 6-0 6-1 defeat by Federer.

"This year has been testing and I've learned a lot, it's just a shame to finish it with another very harsh lesson," says Murray.

"There won't be drastic alterations to my game but it was not good enough, and even against the best players I should be giving myself opportunities to win games, sets and matches.

"My goals remain as high as ever and the next target is already clear - to try and win the Australian Open."

Image source, AFP

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

"This is what's great about tennis. 90% or more of fans would have said Roger Federer would win, me included. For whatever reason Stan has managed to come out and execute his game plan and it's a brilliant first set.

"Much was built around the phenomenal serving but he's got to maintain that. I still think Roger is favourite. He was changing a few things towards the end of that set to try and break Stan's rhythm."

The last service game from Federer saw a change of approach by serving out wide. Well he is not the only one who can mix it up.

Wawrinka tries a different tactic - that of serving tight into Federer's body. And it works. After just after 30 minutes of the match, he has two set points.

And he takes the first one after sending Federer sprinting around the court. Earlier today Novak Djokovic dropped a set for the first time this week. Roger Federer has just copied him.

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

When asked what he would say to Wawrinka having gone from a 5-2 lead to 5-4, Leon said: "I'd remind him that you'd rather be in this position than any other. It's a great position. Then I'd say how did you get yourself in this position? You served great, were dictating play and exposing Federer's backhand. He's got to focus on his game and back it up with aggressive play to the backhand."

Image source, Getty Images

And those in the O2 tonight are being treated to a thrilling match.

Federer throws his serves out wide in the ninth game and it is a successful tactic as Wawrinka struggles to return any of the serves.

The world number two has actually won more points than his opponent, who is again serving for the opening set.

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

"It's interesting to watch Federer employing his match tactics. It's a good lesson for youngsters, to learn how to stop the momentum of your opponent. Dropping back behind the baseline and changing the height and pace of the return. A subtle change of pace can put them off."

Federer wins the first two points off Wawrinka's serve, and then has two break points at 40-15. Wawrinka's first serve finds the net and he then goes too long. An immediate break back from Federer. So, who among you wrote him off? Never write Federer off.

But he still faces a battle to save the first set though.

It is not only singles action at the O2 Arena this week. There have also been two semi-finals in the doubles competition at the ATP World Tour Finals today.

Image source, Getty Images

The pairing of Croatia's Ivan Dodig and Brazil's Marcelo Melo beat Lukasz Kubot of Poland and Robert Lindstedt of Sweden 4-6, 6-4, 10-6.

Dodig and Melo will face the Bryan brothers, Bob and Mike. The Americans defeated the French duo of Julien Benneteau and Edouard Roger-Vasselin 6-0 6-3.

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

"I'm so impressed by how Stan has come out here tonight, to be so loose and confident. I think Roger has been taken a bit by surprise. I don't think he expected Stan to start at this kind of level. It's almost unbelievable. He's playing smart, sticking to the game plan and from that he's taking it on and hitting winners from all over the court."

Not only is he struggling with Wawrinka's serve, Federer isn't having much luck either. The ball clips the top of the net, but unlike Wawrinka's similar shot, which crept over a few games earlier, it drops the wrong side of the net. That brings about another break point for Wawrinka.

But if the world number four had been a bit fortunate with the previous shot, there was nothing lucky about a beautifully-hit winner. Two breaks of serve. Wawrinka serving for the set. Federer in shock.

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

"Spectacular serving from Stan. Power, accuracy and most over 130mph, giving Roger very little chance even though he's one of the best at getting the ball back in play. He's serving very well and that is enabling him to dictate points."

Tremendous serving from Wawrinka. Who already has three aces. Federer is struggling to connect with them, never mind keep them on the court. A lovely backhand down the line, when Federer was expecting a cross-court attempt takes it to 4-2.

Remember, Wawrinka has only won twice in 16 meetings between the pair.

A routine service game is just what Federer needs to calm the situation down. And that is exactly what happens in the fifth game, although Federer is still one break of serve behind in the opening set.