For the fourth consecutive match in the head-to-head history between No. 2 seed Novak Djokovic and No. 4 Juan Martin del Potro, the result wasn't clear until the maximum number of sets had been contested.
This did not deter Djokovic, who has been dialed in since losing in the US Open final to Rafael Nadal. After beating del Potro 6-3, 3-6, 6-3, Djokovic has won 19 consecutive matches.
He took two Davis Cup matches against Canada, then won the titles in Beijing, Shanghai and Paris, beating Nadal, del Potro and Ferrer in those finals.
And now he follows Nadal into the semifinals in London. Del Potro, meanwhile, will have to beat Federer on Saturday if he wants to join them.
For a while, it looked like this one would be over quickly.
Djokovic scored a decisive 4-2 break on a beautiful point, moving the 6-foot-6 Argentine all over the court and finishing with a cross-court forehand that del Potro couldn't track down. He won the first set with an ace down the middle -- on a second serve.
But del Potro, as he always seems to do, rallied. With Djokovic gesturing to his coach, Marian Vajda, that he was having difficulty focusing or seeing the ball, del Potro ran off with the second set.
Djokovic was pressured on his serve early in the third, digging out of a 15-40 hole at 1-all. An ace, a big forehand and two baseline errors from del Potro gave him the game.
Serving at 2-3, del Potro did not have the same success. After he got caught with the wrong grip to force a break point, del Potro's big forehand sailed past the baseline and Djokovic was up 4-2. When the Serb stepped into a gorgeous backhand down the line, it was over.
Over four days, there has been no evidence to suggest that the world's best two players, Nadal and Djokovic, won't meet for the title in London.
Federer on the board
OK, let's see now … six-time champion Roger Federer had won a record 42 matches at the year-end event and his Thursday opponent, Richard Gasquet, had won, um, one.
How do you suppose it went at the O2 Arena?
Yes, even at the doddering age of 32, Federer managed to scrape out a 6-4, 6-3 victory, converting his sixth match point. Make that 43 wins. This sets up the pivotal Saturday match with Juan Martin del Potro, which will determine the player who advances to the semifinals. Federer has reached the final four in London 11 times in 12 tries.
Federer, the No. 6 seed, thoroughly spanked the No. 8 Gasquet, whose second trip to the year-end event did not go well; he's 1-4 in career matches at the year-end event.
Afterward, Federer addressed his difficult season, which has included only one title, on grass, in Halle, Germany.
"I'm not coming back from a serious injury," Federer said, "but I'm coming back from a lot of sort of ups and downs, resetting things, trying out things, making sure I get my confidence back, my movement and so forth. At the end of the day, many of the matches you do play, you're actually not playing your very best. Those are the ones you just have to scrape through sometimes.
"That's the beauty of it. When things go well, it's all rosy. If things don't go so well, you start digging. That's kind of what happened a little bit this year for the media sometimes."
Federer is now 11-2 against the Frenchman and, for what it's worth stands 12-0 in his second round-robin matches. Federer, who has always liked the controlled atmosphere of an indoor court, saved five of six break points and got 75 percent of his first serves in.
Both Federer and del Potro are 1-1 this week, but Federer has a 14-5 head-to-head advantage, including a quarterfinal win (in three sets) last week in Paris.
"I think the win in Paris was big for me, because he did beat me three times indoors straight, the Basel finals twice, and also here last year," Federer said. "I'm glad I got one back in Paris because I knew we could be again in the same group, we could be playing each other. Think that win for me in Paris psychologically was very important."
Bryans survive match point
The No. 1-seeded team of Bob and Mike Bryan ground out a must-win against Aisam-Ul Haq Qureshi and Jean-Julien Rojer to keep its chances alive in the A Group. The score was 7-6 (3), 1-6, 14-12 -- and that wild tiebreaker was every bit as close as the score implies.
The Bryans saved a match point when Rojer's cross-court backhand return sailed wide and needed four of their own before escaping with the victory. They are already the ATP World Tour's No. 1 team for the ninth time in 11 years.
Meanwhile their Group A colleagues, No. 3 Ivan Dodig and Marcelo Melo, qualified for the semifinals with a 6-3, 3-6, 10-2 win over No. 8 Mariusz Fyrstenberg and Marcin Matkowski. Dodig and Melo are 2-0, while the Bryans are 1-1.
Standings through Day 4
Note: Players in each group will meet once each to determine the four semifinalists.
(1) Rafael Nadal 2-0*
(5) Tomas Berdych 1-1
(7) Stanislas Wawrinka 1-1
(3) David Ferrer 0-2
(2) Novak Djokovic 2-0*
(4) Juan Martin del Potro 1-1
(6) Roger Federer 1-1
(8) Richard Gasquet 0-2
* Clinched spot in semifinals
Forward spin: Day 5
It's the final day of round-robin action in Group A and the fates of No. 5 Tomas Berdych and No. 7 Stanislas Wawrinka -- both 1-1 -- hang in the balance.
In the afternoon session, Wawrinka meets No. 3 David Ferrer (0-2). Later, Berdych faces No. 1 Rafael Nadal, who is 2-0 and has already qualified. The match can be seen on ESPN2 at 3 p.m. ET.
In most scenarios, if either Berdych or Wawrinka wins and the other doesn't, the winner is through to the final four. If both players win, depending on the number of sets involved, Berdych would be the more likely semifinalist.
In doubles, two 1-1 teams are trying to qualify. Leander Paes and Radek Stepanek play Marcel Granollers and Marc Lopez (0-2) and then Alexander Peya and Bruno Soares play David Marrero and Fernando Verdasco, who are 2-0 and have already qualified.