By Martyn Herman
LONDON (Reuters) - Roger Federer rediscovered his verve in the nick of time to outclass Austrian Dominic Thiem 6-2 6-3 on Tuesday and maintain his hopes of reaching the last four at the ATP Finals.
The 37-year-old's proud record of always reaching the semi-finals at the season-ender since it moved to London in 2009 looked in peril after a surprise loss to Kei Nishikori in his opening round-robin match on Sunday.
Another defeat would have made it impossible for the Swiss to reach the semis but the six-time champion rebounded in style to post a classy win in which he lost only six points on serve.
There is still work to do for the second seed, however, especially as giant South African Kevin Anderson, who he plays at the conclusion of the group on Thursday, is in red-hot form.
Anderson, the first player from his country to qualify for the ATP's blue riband tournament since 1995, leads the Lleyton Hewitt group with two wins after trouncing Nishikori 6-0 6-1.
The inspired 32-year-old won the first 11 games but was denied only the second 'double bagel' in the event's 48-year history after Federer's 2005 rout of Argentina's Gaston Gaudio.
"I hope it is not a match like Kevin Anderson played this afternoon!" Federer, who lost their Wimbledon quarter-final clash this year, said on court.
"It feels good (today). I am very happy that I showed a reaction after the last match. It's not something I'm used to -- losing then coming back to play."
Federer was erratic and grumpy in his straight sets defeat by Nishikori, but he returned 48 hours later with a majestic display which delighted the sell-out O2 Arena crowd.
"I don't know what happened against Nishikori. Maybe it's the round-robin format that got to me," Federer said.
"So for me it was really important to remind myself what a thrill it is to play here at the O2. I love playing in London, always have, always will."
He certainly looked back in the groove, making only 11 unforced errors, compared to the 34 against Nishikori, and surrendering just six points on serve.
Thiem, who lost his opener to Anderson, saved a break point in the first game but Federer kept him under constant pressure and broke at 1-1 when the 25-year-old sent a forehand long.
Two tame volley errors by Thiem gave Federer another break for a 5-2 lead as he cruised to the opening set.
Federer turned the screw to break again immediately in the second set. Thiem did let rip with some stunning backhands but with the world number three back in the groove they made little impression as the Swiss sealed a 56th match win at the tournament he has qualified for a record 16 times.
Anderson looks almost certain to reach the semi-finals even if he is beaten by Federer on Thursday.
He produced a near-perfect display against Nishikori who he lost to two weeks ago in Paris.
"It's amongst the best (matches) I've played," world number six Anderson told reporters. "I did a really good job constantly applying the pressure and not letting up.
"I felt I did a fantastic job throughout really."
(Reporting by Martyn Herman; Editing by Hugh Lawson and Ken Ferris)