Andy Murray was thrilled to be first out of the blocks as he opened his Barclays ATP World Tour Finals campaign.

He started well with a three-set victory over US Open champion Juan Martin del Potro at London's O2 Arena.

The British number one had the honour of playing the first singles match at the prestigious eight-man event and began in style, winning the first five games before digging deep to seal a 6-3 3-6 6-2 triumph.

"It was a really good start," said the 22-year-old, who reached the semi-finals on his tournament debut last year.

"The start of the match was important. Me and Juan haven't played that much since the US Open so I kind of expected a little bit of a scrappy match maybe.

"He didn't start particularly well but after 5-0 I thought the standard was very good. We played some great points.

"Obviously I'm happy I managed to come back in the third because he was playing well."

The first set was a strange affair, with Murray saving two break points in the opening game before going on to take a 5-0 lead.

Del Potro took a medical time-out for attention to a nosebleed after the third game but he could not stem the flow of errors coming from his racquet.

Having saved two set points to avoid a whitewash, the Argentinian retrieved one of the breaks before Murray finally clinched the opener at the eighth time of asking.

However, the danger signs were there for the world number four and a sloppy service game at the start of the second set allowed his opponent to open up a 3-0 lead.

The topsy-turvy nature of the contest continued, Murray first breaking back after Del Potro's volleying again let him down only for the 21-year-old to power into a 4-2 lead with yet another break.

And three games later the world number five took advantage of his second set point to level proceedings.

Murray was on the back foot but seized his chance at the start of the decider, letting out a huge roar after breaking for a 2-0 lead.

And once again Del Potro eventually proved to be his own worst enemy, successive double faults giving Murray two match points and the Scot clinched victory with a crisp backhand winner.

Del Potro has now lost five of his six matches against Murray and said: "Against Andy, always the match is very tough.

"We play three sets in every tournament, every match.

"Today was a little strange but he played great tennis in the third set. He was a little lucky but you need the luck to win. And today it was with him."

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The second Group A clash of the day featured world number one Federer and dangerous Spaniard Fernando Verdasco, and it was the top seed who came through - but only after a big scare.

Verdasco had not so much as taken a set off Federer in three previous meetings but four points into the match he was a break up.

That proved enough for the world number eight to take the first set as the top seed struggled to find his rhythm and range, and the match was effectively decided in a high-quality second set.

Verdasco finally came unstuck in the 12th game, Federer levelling the match on his first break point. And the decider was one-way traffic as a by-now supremely confident Swiss served out a 4-6 7-5 6-1 victory.

After his terrible start, Federer admitted it was key to the outcome that he took his chance when it came in the second set.

He said: "The whole first set I was struggling to really put normal returns into play. And that can happen at times. I thought he did a good job of mixing it up and playing aggressive at times.

"My first break points came for set points and it was a crucial moment for me because I knew the longer the match went, the more my belief was going up and his was going down. Then in the third, I was able to play more freely."

Verdasco highlighted the 11th game of the second set, when the Spaniard had Federer at 0-30 but an attempted winner missed by inches, as the moment when the match slipped from his grasp.

"This passing shot going out for nothing, tennis is like that," said the world number eight. "After that, my fitness, it started going down. He also got confidence and the match changed."