"I don't know what's wrong," Federer said. "I have to analyze it myself. It was over too soon for me today."

The top-ranked Swiss won in Dubai in March. He has since failed to win at Indian Wells, Calif.; Key Biscayne, Fla.; Monte Carlo, Monaco; and Rome.

Federer often missed routine baseline shots against Volandri, and his body language conveyed little will to compete. He hit only two winners with his forehand, normally his best shot, and made 44 unforced errors to Volandri's 23.

Federer's ability is usually enough, even on bad days, but not this time.

"It happens a lot, but sometimes I get through them," he said.

Third-ranked Andy Roddick also had a rough day, losing 6-0, 6-4 to Juan Ignacio Chela. No problems, however, for Rafael Nadal. The two-time defending French Open champion extended his clay-court winning streak to 74 matches with a 6-2, 6-2 win over 15th-seeded Mikhail Youzhny.

Roddick played his first tournament since injuring his hamstring in the Davis Cup last month. A stomach virus also slowed his preparation before flying to Europe.

"I'm not feeling in as good a shape as I should be," he said.

Chela next will face sixth-seeded Fernando Gonzalez, who beat Nicolas Massu 6-7 (4), 6-3, 6-4.

Youzhny had beaten Nadal in their last two meetings — both on hard courts. Nadal was checked by doctors Thursday for a dizzy spell but appeared untroubled in his night match against Youzhny.

Nadal is aiming to become the first player to win the Rome tournament in three consecutive years. He beat Federer in a fifth-set tiebreaker in last year's final.

In the quarterfinals, Nadal will face fifth-seeded Novak Djokovic, who rallied in the second set to beat last year's Australian Open runner-up Marcos Baghdatis 6-2, 7-5.

Federer dropped his serve twice in the first three games and quickly lost control of the first set. In the second set, Volandri broke to take a 2-1 lead when Federer made four errors.

On Volandri's first match point, Federer hit a forehand into the net.

"I was always running behind the score," Federer said. "The crowd didn't affect me much because nothing happened. It's just a disappointing loss."

Volandri's quarterfinal opponent will be 12th-seeded Tomas Berdych, who routed Jose Acasuso 6-1, 6-2. Fourth-seeded Nikolay Davydenko avoided another upset by edging Potito Starace 4-6, 6-2, 7-5.

Federer arrived in Rome more than a week before the tournament to step up his clay-court training for the French Open, which begins May 27. It's the only Grand Slam that Federer hasn't won.

"I was the first player to arrive. My preparation was perfect," he said. "But let's not forget that Filippo played well today. Give the guy credit."

Roddick wasn't able to dominate with his usually reliable serve, allowing the 23rd-ranked Chela to dictate play from the baseline.

"I served horribly today and that made the rest of it a little tough for me," Roddick said. "First and foremost, though, he just played better than I did. It wasn't even close today."

Roddick served only one ace, late in the second set, and dropped his serve four times. He saved two match points on his serve while trailing 5-3 in the second set, but Chela served out the match at love in the next game.

"Too little, too late," said Roddick, whose performance came in sharp contrast to his straight-set win over 2004 French Open champion Gaston Gaudio on Wednesday.

Roddick had won six of seven previous meetings with Chela, but those all came on the American's preferred hard-court surface.