Defending champion Roger Federer became the first player to qualify for the ATP World Tour Finals last four after defeating David Ferrer in straight sets at the O2 Arena in London.

In a gruelling, error-strewn encounter, Federer prevailed 6-4 7-6 to secure his 70th win of the year and reach the semi-final of a tournament he is seeking to win for a seventh time.

Ferrer will be wondering what if after converting just one out of 10 break point chances, but has another opportunity to himself qualify for the knock-out phase when he takes on Janko Tipsarevic on Saturday.

Roger Federer
Federer is now 14-0 against Ferrer.

Having beaten Tipsarevic and Juan Martin del Potro respectively in their opening round robin matches, both Federer and Ferrer knew victories for either would all but ensure their qualification for the knock-out phase.

Ferrer had continued the momentum from his victory at the Paris Masters win in overcoming Tipsarevic and looked to continue that in the early stages against the defending champion, who he had never beaten in 13 attempts.

The Spaniard forged three break point opportunities in the opening game, but after errors in the opening two points, Federer produced a superb backhand down the line to take the game to deuce.

Successive points followed as the 17-time major champion saved his serve, and he produced a typical response in the next game, breaking Ferrer as his serve and then normally metronomic forehand let him down.

Federer was however unable to stem the tide on his own serve, and faced three more break points in the next game but a combination of Ferrer's angles going slightly askew and the Swiss' superb forehand on the third break chance allowed him to secure a 3-0 advantage.

A love service game from Ferrer helped build the foundations for another attack at Federer's serve, and at the eighth time of asking he was rewarded, as Federer found the tramlines with an attempted backhand down the line.

The six-time ATP World Tour Finals winner recovered from 15-30 in the ninth game to hold a slight lead, and upon producing a superbly timed forehand, forced a triple break-point, requiring just one as Ferrer produced an error on the backhand side.

The World No.5 began the second set like he started the match, forcing more break point opportunities in the opening game, which forced Federer into using soft hands at the net, and then produce an overhead volley to again keep the match in his favour.

A netted backhand prevented Federer from taking a break lead in the second set, but the chance sparked a lull in the action as both players cut down the errors and became more dominant on their own serve.

But just as the second set looked to be heading for a tiebreak, Ferrer froze while serving out. He followed a double fault with a wayward forehand, but two Federer errors, allowed the seven-time winner on the ATP Tour in 2012 to serve out comfortably.

The first mini-break went the way of Federer in the tie-break as Ferrer netted a backhand volley; and then successive aces after the change of ends gave him three match points. The No. 4 seed saved the first two but a long return of serve gave Federer his second win in succession and a place in the knock-out round.