World number one Roger Federer bowed out of the Rome Masters as he suffered a surprise quarter-final defeat at the hands of Radek Stepanek.

Federer lost 7-6 (7-4) 7-6 (9-7), his sixth defeat of an inconsistent season.

With the French Open two weeks away, the defeat is a blow to Federer's hopes of claiming the only Grand Slam title to elude him.

After the first set had gone with serve, Stepanek took control of the tie-break with a volley to go up 6-4, then served an ace on his first set point.

Federer squandered a 4-1 lead in the second-set tie-break, and Stepanek converted his second match point with a first serve that Federer could not get back.

At one point in the first set, Federer won four straight service games without losing a point but Stepanek kept himself in the match with his fearsome serve and volley tactics.

"Usually when I have a lead I don't let it go, so it's quite disappointing," said Federer.

"I played so poorly on the big points.

"He's difficult to play. He's always changing his game up a lot. But I've played him in the past, so it wasn't a surprise. I just wish I played better."

Stepanek said: "He definitely doesn't have the results he was used to in previous years, but the other players are getting better.

"I came to the match with the belief that I can win."

The defeat has robbed Federer of the chance of some valuable match practice, with the French Open getting under way at Roland Garros on 25 May.

It comes just two days after the defending champion Rafael Nadal, who has the upper hand against Federer on clay, was beaten by Juan Carlos Ferrero.