* Defending champion Federer through to last four

* Tipsarevic first player eliminated

LONDON - Defending champion Roger Federer guaranteed his place in the last four of the ATP World Tour Finals with a win over David Ferrer on Thursday but Janko Tipsarevic became the first player eliminated after a second defeat.

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

World number two Federer, seeking 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.

Ferrer is now vying with Juan Martin del Potro for the other semifinal spot from Group B after the towering Argentine pummelled Serbia's Tipsarevic 6-0 6-4.

Tipsarevic, who pulled out of the Paris Masters last week after feeling unwell and hardly practised since arriving, has managed eight games in four sets and cannot reach the semifinals even if he beats Ferrer in his final round-robin match.

"I'm playing horrible, I'm playing worse than horrible, I'm playing the worst tennis that I played in a long time," was the Serb's brutally honest assessment of his tournament so far.

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

Del Potro, beaten by Ferrer in his first match, will probably need to beat Federer in his final group match.

Federer took a while to settle into his smooth rhythm against Ferrer who had numerous opportunities to make inroads in the opening games of their encounter.

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 playing well.