Federer shows his class

Roger Federer is certain to finish the season ranked No.1 in the world after he secured his spot in semi-finals of the ATP World Tour Finals with a three-set victory over Andy Murray at London's O2 Arena.

The British No.1 played superbly to take the first set but Federer was in terrific form and Murray simply had no answer in the second half of the match as the Swiss star triumphed 3-6 6-3 6-1.

Federer and Murray had met three times this year, with the Scot triumphing twice, while he also won in the round-robin stages of this tournament last year.

But from the first point - a superb rally that ended with a Murray mis-hit - the world No.1 seemed determined to avoid a repeat.

The pair traded breaks early on before Murray got the decisive break in the sixth game and the fourth seed clinched the set two games later.

Despite winning the first set, Murray had been under pressure in almost every service game and that pattern continued in the second, with the Scot saving a break point in a lengthy second game.

It seemed inevitable Federer would eventually break through and so it proved in the sixth game, the world No.1 stringing together a series of sublime points.

Despite the best efforts of his opponent, the 28-year-old duly served out the set to level an enthralling match.

The decider promised to be a thriller but it was all Federer, who produced a masterclass to close out the match in emphatic style.

"Playing against Andy is special for all of us here. I thought I played a great match, him as well," Federer said.

"It means a lot to have returned to No.1 and to finish the year again at No.1."

"It was an incredible year for me both on the court and off the court and to be able to break the all-time Grand Slam record and finish the year on top is amazing."

Earlier, Juan Martin del Potro boosted his hopes of qualifying for the semi-finals with a nervy 6-4 3-6 7-6 (7-1) victory over Fernando Verdasco the other Group A clash.

With both players having lost their opening match - Del Potro to Murray and Verdasco to Federer - they knew this contest could make or break their chances.

And, despite failing to serve out the victory, it was the US Open champion who eventually came out on top, meaning Murray can book his place in the last four with victory against Federer this evening.

Verdasco had played superbly for two sets against the world No.1 on Sunday but it could not have been more different today, the Spaniard amassing 11 unforced errors in the opening five games.

Three of those came in the third game and allowed Del Potro to secure the first break of serve.

The world No.5 was erratic against Murray but the power generated by his extra height kept Verdasco largely at bay throughout a first set that only really came to life in the 10th game.

With Del Potro serving for a one-set lead, his opponent suddenly found his range and the Argentinian needed two pinpoint aces to save break points before a Verdasco backhand dropped wide on the second set point.

The initial stages of the second set, although of a better quality, were dominated by serve, but it was Del Potro who cracked first, Verdasco taking