LONDON — Roger Federer gained sweet revenge over Kevin Anderson at the ATP Finals on Thursday but is putting thoughts of his 100th title to the back of his mind after both players reached the last four.

The Swiss broke the giant South African's serve four times in a comprehensive 6-4, 6-3 victory at London's O2 Arena even though Anderson had not faced a single break point in his two earlier round-robin victories.

Second seed Federer suffered a painful loss at the hands of Anderson in the Wimbledon quarterfinals this year after leading by two sets, his only defeat against him in five meetings before Thursday's clash.

The six-time champion started the week with an uncharacteristically ragged display in a straight-sets defeat by Kei Nishikori but has beaten Dominic Thiem and Anderson in straight sets and is looking more like his old self as the tournament progresses.

The victory over fourth-seed Anderson in the Lleyton Hewitt Group takes Federer through to the semifinals for the 15th time in 16 attempts at the season-ending finale and means he will likely avoid a meeting in the last four against in-form Novak Djokovic.

"I'm happy that this is another week like this," said Federer. "Didn't look like it maybe 72 hours ago. But I was able to come back and play good tennis.

"Personally I'm still not thinking of the number 100. I won't let that get in my head, make me go crazy because it should be something I'm excited about and not something I should feel extra pressure.

"As long as I think Novak is in the draw anyhow, he's playing so good again, it's never going to be easy."

The first set was a topsy-turvy affair, with both players quickly into their groove on serve before a succession of breaks.

Federer, buoyed by his customary vocal support, broke to lead 4-3, only to lose his momentum immediately as Anderson leveled.

The Swiss broke the South African in the next game, sealing the break with a crisp backhand volley at the net but was immediately in trouble at 0-40 on his own serve before winning five successive points to take the set.

The second set went with serve until the seventh game, when Federer produced an immaculate sliced backhand that landed on the line to earn a break.

Now playing with freedom, the 37-year-old broke again to seal the victory and guarantee that he tops the group.

Earlier, Dominic Thiem beat Nishikori 6-1, 6-4 but both players will exit at the round-robin stage. Top seed Djokovic is already through to the semifinals after beating Alexander Zverev in the Guga Kuerten Group on Wednesday, with Zverev, Marin Cilic and John Isner battling for the other semifinal berth.

The world No. 1, aiming for a sixth ATP Finals title to pull level with Federer, plays Cilic on Friday while Zverev takes on Isner. — AFP