Spain's David Ferrer plays an overhead smash shot during his men's singles tennis match against Switzerland's Roger Federer at the World Tour Finals in the O2 Arena in London, Nov 8, 2012. [Photo/Agencies]

Endeavor rewarded

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 endeavor with a break in the fifth game when Federer wafted a backhand wide.

"It could have been the other way round, with him leading 3-0," Federer, unbeaten at the tournament since a similar dominant streak was ended by Nikolay Davydenko in the 2009 semifinals, told reporters.

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 landed every first serve to march 6-3 ahead.

Still Ferrer refused to accept the inevitable, saving two match points, but there was no escape as Federer clinched his place in the semifinals for the 10th time in 11 appearances at the elite event with one round-robin match to spare.

"I think he showed why he's so tough to beat today," Federer, who saved nine out of 10 break points, said.

"He makes you hit an extra shot, he makes it physical. You know that mentally he's not going to go anywhere."

Federer has now won his last 12 matches at the ATP World Tour Finals since that loss to Davydenko when the tournament made its debut at the O2 Arena.

With two comfortable wins this year few would bet against him finishing on a high by clinching a hat-trick of titles at the spectacular domed venue alongside the River Thames.

Serbia's Janko Tipsarevic serves during his men's singles tennis match against Argentina's Juan Martin Del Potro at the World Tour Finals in the O2 Arena in London, Nov 8, 2012. [Photo/Agencies]

Tipsarevic served two double faults in the opening game against Del Potro and his evening did not get much better.

Pounding huge forehands into the corners, Del Potro rushed through the opening set in 28 minutes and although the bespectacled Tipsarevic delighted the large evening crowd with a fighting second set the outcome never really looked in doubt.

"I passed the hour mark one more time baby!" Tipsarevic said on his Twitter page soon after leaving the court following a disappointing 76-minute match.

It was hardly the most taxing night for Del Potro and the Argentine remained on court for a little extra practise after his victory with Manchester City soccer player and compatriot Sergio Aguero who had been watching from the stands.

Three years ago when Del Potro reached the final another Manchester City striker, Carlos Tevez, provided a hitting partner while he waited to find out if he had qualified for the semifinals.

Group A will untangle itself on Friday when world number one Novak Djokovic plays Tomas Berdych and Andy Murray takes on winless Jo-Wilfried Tsonga.

All four players can still qualify for the semifinals.