Roger Federer admits earning the year-end world number one ranking is no longer a realistic ambition despite his remarkable renaissance this year.

Federer is already certain to finish behind Rafael Nadal in the final rankings of 2017, but the world number two appears poised to push his old rival for top spot next year.

If Federer wins this week's ATP Finals -- he is through to the semi-finals -- he will finish the year just 140 points behind Nadal.

Having won the Australian Open and a record eighth Wimbledon title this year, Federer could have been forgiven for targeting another spell at number one.

But the 36-year-old has already spent a record 302 weeks in his career in pole position and he doesn't believe the year-end crown is a target worth chasing hard because of the strain it would put on his body.

Federer has a carefully chosen schedule in which he doesn't play as many tournaments as his rivals in a bid to peak at the Grand Slams.

It worked to perfection this year and he said: "Look, it's not a realistic goal in some ways, the world number one.

"It is interesting. It's the ultimate achievement in tennis in some ways, it always has been for me.

"But at this age, it just can't be because I think I'll make mistakes if I start chasing it.

"I'm not sure how much the body allows me to chase goals like this.

"Maybe if I start thinking about it too often and too much, I think I'm maybe also not playing the way I'm supposed to be playing.

"Maybe I tense up, maybe I'm nervous, maybe that's not good for my back. Who knows what it is."

Federer could overtake Nadal early in the 2018, although as the Australian Open champion, he will be defending 2,000 points in Melbourne in January, meaning he would need a strong showing in the year's first Grand Slam to put pressure on the Spaniard.

The 19-time major winner's decision to skip the recent Paris Masters ended his number one bid this year, but he says he has no qualms about that decision.

- No regrets -

"Regrets? I don't have any. But maybe losing to Tommy Haas (in Stuttgart) and Evgeny Donskoy (in Dubai) ended up haunting me," he said.

"I had match points in both matches. It's not like I didn't try.

"So I'm just happy I'm playing a great season. I'm so happy that I was able to reach this level of play and still being able to play also at the end of the year.

"I totally exceeded my expectations. I'm just happy that I'm injury-free and healthy right now and enjoying myself still."

With a seventh ATP Finals title in his sights as he prepares for Saturday's semi-finals, Federer is more focused on the present that any future goals, or the constant speculation about when he will retire.

Quizzed on whether he would still be playing at the 2020 Tokyo Olympics, he admitted he didn't realise he would need to play at least two Davis Cup ties for Switzerland to be eligible for the Games.

"I didn't even know the rule, so thanks for the information," he said.

"I haven't officially retired from the Davis Cup, so not because of that reason.

"It's just because it's always highly unlikely that I'll play at this stage of my career.

"I haven't set Tokyo Olympics as a goal either. I'm not thinking that far ahead."

source: AFP