Two-time Grand Slam champion Andy Murray has claimed that his fellow professionals never doubted Rafael Nadal's ability to perform, despite the Spaniard's recent slump.

Nadal had not beaten any of the so-called 'top four' this year until he executed a straight-sets win over Murray at the ATP World Tour Finals in London's O2 Arena this afternoon.

Murray believes that the media doubted Nadal's threat due to his opponent's inconsistent season, but the players are always wary of the 14-time Grand Slam champion.

"Obviously for a few months this year he was struggling for form a little bit, but I think almost every player goes through that at some stage during their career," Murray told reporters.

"I think a lot of the media had a lot of doubts about him. I think a lot of the players still felt, providing he stayed injury-free, that he would raise his game again and compete at the top of the game. So I'm not really surprised by that."

Nadal will reach the semi-finals if Stanislas Wawrinka wins one set in his match against David Ferrer this evening.