TENNIS/ATP WORLD TOUR FINALS:ARGENTINA'S JUAN Martin del Potro boosted his hopes of qualifying for the semi-finals of the Barclays ATP World Tour Finals at London's O2 Arena with a nervy 6-4 3-6 7-6 (7/1) victory over Fernando Verdasco in yesterday's Group A clash.

With both players having lost their opening match – Del Potro to Andy Murray and Verdasco to Roger 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.

Verdasco had played superbly for two sets against the world number one on Sunday but it could not have been more different yesterday, 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 number five 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 the first break point of the set.

That left the world number eight serving to level proceedings, and he completed the task with aplomb, wrapping up a love game with an ace.

Perhaps realising he had let his opponent into the match, Del Potro upped the ante at the start of the decider.

Verdasco resisted the pressure initially but in the sixth game an inopportune double fault gave his opponent three break points. Two were saved wthen on the third Del Potro stopped the rally to challenge a shot he thought had gone long.

He held his head in his hands awaiting the replay but the ball had indeed missed the line and the Argentinian had the breakthrough.