ROME -- What was once almost unthinkable in tennis now happens more and more often: Roger Federer loses.
The latest defeat came Friday, a 7-6 (4), 7-6 (7) decision to 27th-ranked Radek Stepanek in the Rome Masters quarterfinals.
This was Federer's sixth loss of the year. The top-ranked Swiss lost only nine matches in all of 2007.
"Usually when I have a lead I don't let it go, so it's quite disappointing," Federer said. "I played so poorly on the big points."
Federer piled up a string of defeats at the beginning of this year when he was diagnosed with mononucleosis. He won his first title of 2008 at the Estoril Open last month, the longest he's waited in nine years.
"He definitely doesn't have the results he was used to in previous years," Stepanek said. "But the other players are getting better. I came to the match with the belief that I can win."
The Czech celebrated his victory by writhing like a caterpillar, with his belly on the clay.
"That was the way I celebrated the first tournament victory of my career," he said. "Everyone was asking me when I would do it again, but I was waiting for the right occasion, and today was special."
"It shows how the [other] players are getting better. Everybody is hungry," Stepanek said. "Two players can't win all the tournaments."
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 never won. He has also never won a title in Rome. Last year, Federer was upset in the third round by Italian wild card Filippo Volandri.
Roddick also reached the semifinals in his debut at the Foro Italico in 2002, losing to Tommy Haas. His semifinal opponent this time will be Stanislas Wawrinka, who rallied past eighth-seeded James Blake 6-7 (5), 7-6 (5), 6-1.
The last American to win in Rome was Andre Agassi in 2002.
Roddick improved to 8-0 against Robredo, rushing the net on both his first and second serves against the Spaniard.
"He's a better clay-court player than I am, so I just attacked and, let's face it, I had to volley better than I have in my whole life," Roddick said.
In the third-set tiebreaker, Roddick overwhelmed Robredo with two aces and converted his first match point after following his second serve to net. Federer surrendered a 5-2 lead in the second-set tiebreaker, and wasted a set point at 6-5 on Stepanek's serve by hitting into the net.
Federer had won four of his previous five meetings with Stepanek, but the pair are now 2-2 on clay.
"He's difficult to play," Federer said. "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 converted his second match point with a first serve that Federer couldn't return. In the first-set tiebreaker, Stepanek took control with a volley to go up 6-4, then served an ace on his first set point.
After his illness, Federer responded by winning Estoril and reaching 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 of the first set, Federer won four straight service games without losing a point. Still, Stepanek kept up the pressure with his serve and volley.
"If you let him play his game, he can move the ball all around the court," Stepanek said. "I wanted to keep the pressure on."