By Martyn Herman
LONDON (Reuters) - Swiss great Roger Federer made sure he will be among the last four at the ATP Finals this weekend after outplaying South Africa's Kevin Anderson to top his group on Thursday.
The Swiss once again brushed aside the passing of time with a vintage display, beating Anderson 6-4 6-3 to reach the semi-finals for the 15th time in 16 appearances at the event.
Federer walked out into a jam-packed O2 Arena knowing any repeat of his insipid performance on Sunday against Japan's Kei Nishikori could result in an ignominious early exit.
Displaying the old swagger, however, Federer turned on the style to avenge this year's shock Wimbledon quarter-final defeat by Anderson in which he squandered a match point.
Federer and Anderson both ended up with two wins in the Lleyton Hewitt group but the record six-time champion grabbed first place on the head-to-head -- meaning he is likely to avoid world number one Novak Djokovic in Saturday's last four.
Djokovic will top the other group unless he loses to Marin Cilic and John Isner beats Alexander Zverev.
The tournament has so far lacked thrills but a Sunday showdown between old warriors Federer and Djokovic is still on.
Earlier on Thursday Dominic Thiem beat Nishikori 6-1 6-4 -- a result that meant the Austrian would reach the semi-finals if Federer lost to Anderson and won fewer than six games.
Thiem admitted he was not holding his breath and from the moment Federer took the first set it was just a case of whether or not the second seed would top the group.
"Very happy, the first match against Kei was tough, I never got going," Federer, who also beat Thiem, said on court.
"With my back against the wall maybe it's easier for me to play. I played some good tennis today and I'm thrilled, excited to be in the semis."
A confident Anderson pushed hard early on and had a glimmer of a chance in the sixth game but Federer held with a sublime backhand winner that had the crowd roaring their approval.
Anderson double-faulted twice in the next game to drop serve but Federer wobbled, sending a forehand wide to hand the break back. The Swiss immediately broke again though and switched on the afterburners to clinch the set in the next game from 0-40.
Federer pounced in the seventh game of the second set and claimed victory on his third match point.
Anderson's consolation is that he becomes the first South African to reach the semis in the tournament's 48-year history.
"These round-robin formats are not straightforward," Federer said. "Normally, you lose, you leave. I think it was also difficult for Kevin because he knew he had already qualified."
The day began at the O2 Arena with a news conference to launch the new ATP Cup -- a team event that will begin in Australia in January 2020, just six weeks after tennis's governing body the ITF's revamped Davis Cup finals.
While the 118-year-old Davis Cup faces the squeeze from a new rival, Federer, at least, marches on.
(Reporting by Martyn Herman; Editing by Toby Davis and Ken Ferris)