Nadal wary of Berdych

"I lost to him in Cincinnati last year and in Canada this year. So it's going to be a tough match," Nadal said of the 21-year-old Berdych.

"I don't think there is a favourite player. I don't feel myself as a favourite player."

The World No. 2 has lost his last two matches to Berdych at the Toronto Masters and the Cincinnati Masters, though he beat the 21-year-old the last time they played on clay at last year's final in Bastad.

Nadal is the last Spaniard left in what has proven a disappointing series for the hosts, as David Ferrer and Juan Carlos Ferrero were eliminated in the second round and Tommy Robredo got bounced in round three.

The twice French Open champion won in Madrid last year on his favourite surface, but refuses to take Berdych for granted.

"The two times I've played him, my level was quite low and I was feeling quite tired. Cincinnati was an incredible match. Toronto I didn't play well, it was very windy."

"But I also need to say that he's a player that pushes you, puts pressure on you. He plays very aggressive from behind the baseline. So you cannot give him initiative because if you do, you're dead."