LONDON  - Roger Federer clinched a place in the semi-finals of the ATP Tour Finals on Thursday as the defending champion ground out a 6-4, 7-6 (7/5) win over Spain's David Ferrer at London's O2 Arena.

Federer required a straight set win over the fourth seed to seal his spot in the last four with one Group B match to spare and he reached that goal despite a scrappy performance that included 39 unforced errors. While Federer was some way from his best, the world number two displayed his priceless knack of elevating his game on the big points as the 31-year-old extended his winning run at the Tour Finals to 12 matches.

As well as securing his semi-final berth, Federer also achieved a notable personal landmark by recording his 70th match win of the year -- the first time he has done that since he was at the very peak of his form in 2006.

It was a familiar story for Ferrer, who has now lost all 14 of his meetings with Federer, and the Spaniard will need a victory over Janko Tipsarevic in his final match on Saturday to have any chance of qualifying for the semi-finals. Going into this match Ferrer had won only three of 30 sets against Federer and, although he walked on court on an 11-match unbeaten run that brought him titles at the Valencia Open and Paris Masters, the 30-year-old was soon playing second fiddle to the Swiss yet again.

Presented with a golden chance to secure a vital break in the first game, Ferrer was unable to convert as Federer wriggled out of trouble three times. With Ferrer still cursing that missed opportunity, Federer went for the kill and pressured the Spaniard in a crucial mistake, an off-balance backhand that sailed long, to break for a 2-0 lead.

This was Ferrer's 12th match in 16 days and he could have been forgiven for waving the white flag when three more chances to break slipped through his fingers during a marathon third game. To his credit he refused to surrender and earned two more break points in the Swiss star's next service game, finally converting one at the eighth attempt when Federer pushed a backhand wide.

But, despite struggling badly on his first serve, Federer was still able to wrestle the set away from Ferrer with a ruthless break to love at 5-4. That burst of brilliance from Federer was followed by another dip and he had to stave off two break points in the first game of the second set. Ferrer remained a nuisance for Federer, who was throwing in enough unforced errors to keep the Spaniard in with a chance, and it needed a tie-break to decide the set.

However, Federer, shrugging off his inconsistent spell, finally saw off the dogged Ferrer with a pair of brilliant forehand winners to settle the contest.