ROME — Roger Federer defeated towering Ivo Karlovic 7-6 (4), 6-3 today to reach the quarterfinals of the Rome Masters.

The top-ranked Federer improved to 6-0 in his career against the 6-foot-10 Croat.

"It's difficult to judge against him. Ivo has an unbelievable serve, probably the best we have in the game," Federer said. "And he comes to the net too, so there's always a lot of pressure."

Karlovic served 12 aces, routinely registering above 136 mph.

Federer tried to lob Karlovic early on, but eventually discovered that his best solution was to aim for passing shots.

In the first-set tiebreaker, Federer took control with a return winner off Karlovic's first serve to go up 6-4, then closed out the set by making Karlovic reach too far for a volley.

Federer landed a forehand on the line to finally break Karlovic's serve and take a 3-2 lead in the second set, then broke again to close out the match when Karlovic missed a volley.

Federer will next face Radek Stepanek, who defeated Peruvian qualifier Luis Horna 6-4, 1-6, 6-3.

Novak Djokovic also advanced with a 6-3, 3-6, 6-3 win against Igor Andreev.

Djokovic stopped taking medication for strep throat on Tuesday, and he appeared sluggish in the second set on a warm day at the Foro Italico.

"I knew it was going to be a tough match," Djokovic said. "He plays his best tennis on clay."

At the Monte Carlo Masters last month, Djokovic pulled out of his semifinal match against Federer, citing dizziness and a sore throat.

The Australian Open winner will now play Spanish clay-court specialist Nicolas Almagro, who advanced when Fernando Gonzalez pulled out with a right leg injury.

Gonzalez was the runner-up to Rafael Nadal last year and was coming off a title last weekend in Munich, Germany.

Three-time defending champion Nadal was beaten in straight sets by 2001 champion Juan Carlos Ferrero on Wednesday.

"I feel bad for Rafa," Federer said. "He's had such a great run the last few years. Now there's a chance for some other players."

Ferrero, the only former Rome winner still in the draw, was to play Stanislas Wawrinka.

Also, Andy Roddick faced rising local favorite Simone Bolelli of Italy.

This tournament is a clay-court tuneup for the French Open, which begins May 25.

Federer improved his season record on clay to 11-1, with his only loss coming to Nadal in the Monte Carlo final.

Rome is one of only three Masters Series tournaments Federer has never won, along with Monte Carlo and Paris. He was runner-up to Felix Mantilla in 2003 and lost a fifth-set tiebreaker to Nadal in the 2006 final.

Djokovic improved to 4-0 in his career against the 26th-ranked Andreev, who committed six double faults and made 20 unforced errors in the first set.

"I hope I can recuperate physically for tomorrow," Djokovic said.