* Federer through to last four
* Defending champion beat Ferrer in straight sets
* Tipsarevic first player eliminated (Adds detail)
LONDON, Nov 8 (Reuters) - 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.
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 semi-final 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 semi-finals 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.
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.
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.
"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 semi-finals, 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 semi-finals 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.
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 semi-finals.
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 semi-finals. (Editing By Alison Wildey)