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 yesterday and will not be guaranteed a spot in the semi-finals even if he beats Verdasco.

Since returning to the Tour this month after six weeks out with a wrist injury, Murray has won the first set of all his matches but then sometimes struggled to polish off his opponents.

At the Paris Masters two weeks ago he won the first set against Radek Stepanek 6-1 only to lose the next two and yesterday his serving let him down as he faded to a 3-6 6-3 6-1 defeat at the hands of Federer.

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.

"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 yesterday 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."

Federer faces a rematch against Del Potro, whose five-set victory over the five-time defending champion in the final of the US Open in September provided one of the biggest stories of the year.

The Swiss, too, is not thinking about the qualification scenarios and is looking forward to locking horns again with the big-hitting Argentinian, who has struggled with form and fitness since his Flushing Meadows triumph.

"It could be a make-or-break match for us to get through," said Federer. "If that's the case, I'll give it all I have and try to beat him this time.

"Last time we played, it was a fantastic match in New York. And it seems like he's playing well, whereas I thought he might have some issues maybe with his wrist or with his abdominal strain. But he seems fine now. So he looks like he's getting better as the tournament goes on.

"I don't think the players think too much, are we through or not? So many times we have had the situation that the player needed something and the other thing happened. Matches are always played full on."