Roger Federer (AP)

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London - Defending champion Roger Federer maintained his 100% record at this year's ATP World Tour Finals by completing his group fixtures with a 6-1, 3-6, 6-3 victory over America's Mardy Fish on Thursday.

Federer was already guaranteed top spot in Group B and a place in the semi-finals after winning his first two matches and the Swiss survived a mid-match wobble against Fish to extend his impressive run at London's O2 Arena.

Federer's 37th career Tour Finals victory moved him into second place on the all-time list ahead of Boris Becker and he is only two wins behind leader Ivan Lendl.

His semi-final opponent is still to be decided, but if Federer can maintain this impressive form it would be no surprise to see him win the tournament for a record sixth time and equal Lendl's tally of victories in the process.

In contrast, Fish, who was already out of contention for a semi-final spot after losing against Rafael Nadal and Jo-Wilfried Tsonga, ends his first visit to the season finale without a win.

While Andy Murray is out of the tournament due to a groin injury and Rafael Nadal and Novak Djokovic are weary at the end of a gruelling campaign, Federer, who is now on a 15-match winning run, looks fresher than ever after a break following the US Open.

Even by his sky-high standards, Federer's 6-3, 6-0 demolition of Rafael Nadal on Tuesday ranked as one of the best performances of his life and he carried on in the same vein in the first set against Fish.

The 30-year-old's piercing forehands repeatedly pushed Fish behind the baseline as the American scrambled to stay in the points and it was no surprise that Federer broke twice in the first four games to take a 3-1 lead.

Even when Fish earned three break points when Federer served for the set, the 16-time Grand Slam winner produced a barrage of brilliant serves to get out of trouble.

Fish had nothing to play for and could have been forgiven for bowing out quietly, yet he refused to surrender and came out swinging at the start of the second set.

His aggression seemed to take Federer by surprise. Federer managed to save one break point, but Fish kept coming at him and forced a careless error that gave the 29-year-old a break in the fourth game.

Fish's formidable serve kept Federer from wrestling back the initiative in time to save the set.

But Federer has too many weapons at his disposal to be subdued for long. A sublime drop shot gave him two break points and he converted the second to move 2-0 ahead in the final set.

Federer's concentration may have wavered for a while in the middle of the match, but he was fully focused now and comfortably served out the win.