Andy Murray is determined to start taking advantage of his fast starts after admitting he is letting his rivals back into matches.

The British number one will take on Fernando Verdasco on Thursday afternoon in his final round-robin match of the Barclays ATP World Tour Finals at London's O2 Arena. Murray beat Juan Martin del Potro in three sets on Sunday but let a lead slip against world number one Roger Federer and will not be guaranteed a spot in the semi-finals even if he beats Verdasco.

Murray said: "The one thing that's been a problem for me since I came back was, I played well-ish the first set of the matches, then when the adrenaline wears off there's not quite the same spark in the legs that there was at the start of the year."

He added: "I've had letdowns in a lot of matches. I've let guys back in the match straightaway, early in the second set. And that's something that I was doing really, really well when I was playing a lot at the beginning of the year. When I was getting ahead, I was not giving guys chances to come back."

Federer's victory means all four players in Group A can still make the last four, although wins for Murray and the world number one would send them both through.

The Scot, though, is just focusing on giving himself the best chance by beating Verdasco.

"The only important thing is to try and qualify from the group, whether you win one match, three matches, two matches," he continued.

"Last year I won three, lost in the semis. This year I could potentially lose two matches and win the tournament. So you just never know.

"The match against Verdasco, obviously I'll try and win. If I do that, there's a good chance I go through. If I win in straight sets, then, well, a great chance that I go through."