ROME — Roger Federer lost again, falling to 27th-ranked Radek Stepanek 7-6 (4), 7-6 (7) Friday in the Rome Masters quarterfinals.

Once seemingly invincible, Federer was beaten for the sixth time this year. The top-ranked Swiss lost only nine matches in all of 2007. He piled up a string of losses at the beginning of this year when he was diagnosed with mononucleosis.

Stepanek advanced to the semifinals against the winner of the night match between Australian Open winner Novak Djokovic and Spanish clay-court specialist Nicolas Almagro.

In the other half of the draw, Stanislas Wawrinka rallied past eighth-seeded James Blake 6-7 (5), 7-6 (5), 6-1 to reach his second semifinal in two weeks. Wawrinka will next face No. 6 Andy Roddick or No. 14 Tommy Robredo.

The Rome Masters is a clay-court tuneup for the French Open, which begins May 25. Roland Garros is the only Grand Slam Federer has not won. He has also never won a title in Rome. Last year at Foro Italico, Federer was upset in the third round by Italian wild card Filippo Volandri.

Federer's elimination comes two days after three-time defending champion Rafael Nadal was beaten by Juan Carlos Ferrero. Nadal said he was not at his best because of a foot blister.

Federer had won four of his previous five meetings with Stepanek, but the pair are now 2-2 on clay.

In the first set tiebreaker, the Czech took control with a volley to go up 6-4, then served an ace on his first set point. Federer surrendered a 4-1 lead in the second-set tiebreaker, and Stepanek converted his second match point with a first serve that Federer couldn't return.

After his illness, Federer snapped back by winning the Estoril Open last month for his first title of 2008. He also reached the final of the Monte Carlo Masters in his last outing, losing to Nadal.

Federer pronounced himself fully fit at the beginning of this tournament, and his problem Friday didn't appear to be physical.

At one stretch in the first set, Federer won four straight service games without losing a point. Still, Stepanek kept up the pressure by serve-and-volleying and winning 25 of 41 points at the net. Stepanek served eight aces to Federer's seven.

Blake was coming off three-setters in his opening two matches, and the American appeared to lose energy as his match with Wawrinka wore on.

Blake missed an easy volley to hand Wawrinka a break in the first game of the third set, and Wawrinka rolled from there.

"It's my first Masters Series semifinal, so I'm very pleased," Wawrinka said.

Wawrinka called for a trainer to treat his lower back with a 6-5 lead in the second set, then again with a 2-1 lead in the third, but the problem did not appear to affect his play.

Wawrinka is ranked a career-high 24th this week after reaching the semifinals of the Barcelona Open last weekend, losing to David Ferrer.