London: A few days after it appeared as though a Roger Federer impersonator had gatecrashed the O2 Arena, the Swiss great resumed normal service to outplay Kevin Anderson and reach the last four of the ATP Finals on Thursday.
Federer walked out into a jam-packed O2 Arena knowing any repeat of his insipid performance on Sunday against Japan's Kei Nishikori could have resulted in an ignominious early exit.
But the 37-year-old again shrugged off the passing of time to swagger past the dangerous Anderson 6-4, 6-3 and reach the semi-finals for the 15th time in 16 appearances at the event.
Record six-time champion Federer turned on the style to avenge this year's gut-wrenching 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 Swiss grabbed first place on the head-to-head record - 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.
A tournament that has so far lacked thrills could yet have a Sunday showdown between old warriors Federer and Djokovic.
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 the second seed or Anderson would top the group.
"Very happy, the first match against Kei was tough, I never got going," Federer, who rebounded from an ugly performance against Nishikori to beat Thiem in his second round-robin match in the Lleyton Hewitt group, 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 from 0-30 with a sublime backhand winner that ripped past his opponent and had the crowd roaring its approval.
Shaken, Anderson double-faulted twice in the next game to drop serve but Federer wobbled, sending a forehand wide to hand the break back. The 20-time Grand Slam champion 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 as the world number six netted a forehand.
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, is showing few signs of wear as he marches on towards his 100th career title.
"I'm happy that this is another week like this," he said. "Didn't look like it maybe 72 hours ago. But I was able to come back and play good tennis."