Andy Murray blamed poor serving rather than the distraction of a mid-match haircut for his chastening loss to a resurgent Rafael Nadal at the ATP World Tour Finals.

The internet was abuzz after Murray sat down at the change of ends following the third game, took a pair of scissors out of his bag and snipped his fringe.

He withstood Nadal pressure until the 10th game of the first set but a run of five games in a row for the Spaniard put him on the way to a 6-4 6-1 victory.

It was certainly not a good performance mentally from Murray, who was tetchy throughout and committed far too many errors, particularly in the second set.

But he denied cutting his hair was symptomatic of any loss of focus ahead of the Davis Cup final next weekend.

The 28-year-old said: "I don't know why such minor things make such a big deal to you guys.

"I had some hair in my eye, and I just wanted to get rid of it. That literally took two seconds. That was it. It was nothing to do with next week or anything to do with the outcome of this match.

"I started the match extremely well, I think. I played a very good first game. Although I got broken in the second game, it was a good game. I was hitting the ball well.

"I held all the way through to 4-4 from that change of ends. It wasn't like it was something that affected me at all after it happened."

Nadal did not see the incident but reacted with surprise when told about it later.

With eyebrow raised, he said: "It was not the right time to know that (his hair was too long). He has practice before to try to know that. It's a good solution."

The result means Murray is likely to need to win his final group match against Stan Wawrinka on Friday to progress to the semi-finals, while he still requires one more victory to guarantee that he will finish the year ranked number two.

Murray's performances at the O2 Arena have been a mixture of the good, the bad and the ugly, and he has never won both of his opening two group matches since the event moved to London in 2009.

The Scot had been impressive against David Ferrer on Monday despite spending much of his preparation time playing on clay ahead of the Davis Cup final.

He began well here, breaking Nadal in the opening game. It was a poor game from the Spaniard but he quickly recovered and showed his growing confidence is well placed.

Nadal will finish the season without a grand slam title for the first time in a decade but he has been looking much more like his old self in recent weeks and began to turn the screw on Murray.

Nadal was playing aggressively, dictating with his forehand and keeping Murray firmly on the back foot.

The Scot fought off break points in the sixth and eighth games but his defences were breached as Nadal broke to love to take the opening set, and two successive double faults from Murray saw him broken again at the start of the second.

Murray only made 43 per cent of first serves and won just 34 per cent of second-serve points.

And, although he was impressed by Nadal's form, Murray conceded he had not helped himself.

"He obviously played better today than he did when I played him earlier in the year," said Murray, who produced one of the most eye-catching results of his career by beating Nadal on clay in the final of the Madrid Masters.

"Also I didn't help myself out there today. I served extremely low percentage, maybe lowest percentage I served the whole year in any match. That's not good enough against someone as good as Rafa."

This was the first time Nadal had beaten a player ranked in the top two since the French Open last year.

The fifth seed, who extended his lead over Murray to 16-6, said: "It is an important victory obviously because that puts me in a good position to try to be in the semi-finals, and at the same time I have a big day against a great player on a tough surface."