Roger Federer is two wins away from clinching his 100th title on the tour but the tennis great insists he is not going to let the milestone mark get into his head at the ongoing ATP Finals in London

Seeded second, Federer booked his berth in the semi-final of the year-ending tournament for the 15th time in 16 appearances on Thursday with a straight-set win over fourth seed Kevin Anderson.

The Swiss great was starting at an early exit after his opening-round loss to Kei Nishikori but he heads into the last-four round as the topper of the Lleyton Hewitt group following a remarkable comeback.

Federer lost an opportunity to reach the milestone at Paris Masters earlier this month as he was ousted by world number one Novak Djokovic in the semi-final. He will be determined to go the distance this week in London but another meeting with the in-form Serb is on the cards for the seasoned campaigner.

"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 [to earn]," Federer was quoted as saying by ATP's official website.

"It's just going to be hard to finish it. I'm happy I gave myself the opportunity. I'm happy that I'm raising my level of play throughout this week. This is what I hope to do. It's exciting to be in this situation now, of course, no doubt."

Roger Federer
Roger Federer is two wins away from clinching his 100th career title.GLYN KIRK/AFP/Getty Images

Federer revealed on Tuesday that he chose rest over training session the day after he was handed a thrashing by Nishikori. The decision worked wonders for the 37-year-old who went on to make light work of Dominic Thiem in his second round-robin match of the tournament.

The 20-time Grand Slam champion added he has never skipped training on the eve of tennis matches in the past but insisted he is happy that rest and family time in between matches is working in his favour.

"I'm feeling great. I'm very happy that I still have energy left in the tank. Mentally, I feel fresh," Federer added.

I didn't think of taking a day off, to be honest, after the Nishikori match. I thought, 'What are we going to do? Where are we going to train?'"

"It's one of the first times I've done it like that. I'm happy it's paying off so far. But I've got to be very, very focused the moment I step on court for the practice, the warm-up. I think that's key, as well."

Federer is likely to avoid a meeting with Djokovic in the semi-final as the Serb looks certain to top the Guga Kuerten group.

While Djokovic has sealed the last-four berth, Alexander Zverev, Marin Cilic and John Isner are all in contention for a place in the knockout round.