Defending champion Roger Federer guaranteed his place in the last four of the ATP World Tour Finals with a straight-sets win over Spain's David Ferrer on Thursday.

The Swiss world number two, bidding for a record-extending seventh title at the end-of-season finale, dished out yet more punishment to Ferrer in a 6-4 7-6 victory at the O2, taking his career record over the tenacious Spaniard to 14-0.

Federer took a while to settle into his smooth rhythm, though, and Ferrer, who beat Juan Martin del Potro in his opening match, had numerous opportunities to make inroads in the opening games of their Group B clash.

Ferrer was 0-40 ahead on Federer's opening service game but having failed to capitalise he was immediately broken himself.

He squandered more break points in the next game as Federer found himself 3-0 up without even playing well.

Missed opportunity

Ferrer, who has a season-leading 73 match wins this year, could have been forgiven for feeling pessimistic about his prospects but he was finally rewarded for his endeavour with a break in the fifth game when Federer wafted a backhand wide.

Having hauled himself back on level terms, however, Ferrer wavered at 4-5, allowing Federer to take the opening set.

Ferrer, darting around the court in his fluorescent lime green top, hung on to Federer's coat tails in the second set to extend the Swiss maestro into a tiebreak.

Federer's serve then clicked into gear after something of an off day and he marched into a 6-3 lead.

Still Ferrer refused to accept the inevitable, saving two match points, but there was no escape as Federer clinched his place in the semi-finals with one round-robin match to spare.

"I'm extremely happy, it's a tough match. David's been on a great run, winning in Paris was so deserving," Federer said on court.

"It was a long match and I'm extremely happy.

"Ferrer is so respected by the fellow competitors. I enjoy playing against him, not because the 14-0 record but because he's a great guy and a great competitor."

Ferrer's fate will be decided in the final round of group matches on Saturday when he faces Janko Tipsarevic.

Should he win that and then go on to reach the final he would replace compatriot Rafa Nadal as world number four.

Federer has now won his last 12 matches at the ATP World Tour Finals since losing to Nikolay Davydenko in the 2009 semi-finals when the tournament made its debut at the spectacular O2 Arena.

With two comfortable wins this year few would bet against him finishing his year on a high by clinching a hat-trick of titles here.