Roger Federer remains on course for his first title of 2008 after defeating unseeded Jose Acususo to reach the quarter-finals of the Sony Ericsson Open.

The Swiss top seed defeated Argentinian Acasuso 7-6 (7/5) 6-2 on Tuesday in one of eight fourth-round matches at the hardcourt event.

World number one Federer has not gone this far into a season without winning a championship since 2000 - the last year he failed to win a title.

Next up for Federer will be American Andy Roddick, who rallied for a 4-6 6-3 6-2 victory over France's Julien Benneteau.

Winning here would be nothing new to Federer, who claimed back-to-back titles from 2004-05.

The 26-year-old reached the fourth round last year, but was upset by Guillermo Canas.

Second seed Rafael Nadal, who lost to Federer in the 2005 final, also advanced to the quarter-finals when he defeated 15th-seeded Frenchman Paul-Henri Mathieu 6-4 6-4.

The Spaniard now owns a commanding 8-0 lead in the all-time series with Mathieu, who retired from their previous match at the Australian Open in January.

Like Federer, Nadal has yet to win on tour this season.

Ninth-seeded American James Blake claimed a 6-3 6-4 win over Radek Stepanek.

Roddick - the American number one and sixth seed - joined his compatriot in the round of eight later by overcoming Benneteau, who had upset countryman Jo-Wilfried Tsonga on Monday.

Taking advantage of a key double-fault by Benneteau, Roddick broke the Frenchmen in the eighth game of the second set for a 5-3 lead before serving out the set.

In the third set, Roddick broke Benneteau twice to take a 3-0 lead before rain briefly suspended play. The break only delayed the inevitable, as Roddick served out the match.

Roddick, who made headlines of his own Monday by announcing his engagement to swimsuit model Brooklyn Decker, won here in 2004.

Making his ninth straight appearance, he will have to overcome long odds against Federer, who is 15-1 in their all-time series.

Fourth seed Nikolay Davydenko, competing in Key Biscayne for the seventh time, seeks his first quarter-final appearance here when he faces unseeded Croat Mario Ancic.

Davydenko hopes to join fellow Russian Igor Andreev, who advanced with a 6-4 7-6 (8-6) win over Canas.

Russia's Dmitry Tursunov, however, failed to move through as 10th-seeded Czech Tomas Berdych claimed a 6-2 6-2 win in their fourth-round match.

Federer felt he was starting to find his form, adding that he has never cruised through the opening rounds.

"Now I feel like I'm playing well," Federer said. "You can tell, as well, I'm moving well and playing well and serving well. That's what it comes down to for me.

"All these matches are a grind. If you look back you always probably thought I won every match 6-2 6-3," Federer said. "It wasn't like that. I had many matches I came through in a close way, and that's what made me the best player for so long, is winning those big points over and over again."

Roddick, who has been assigned the task of stopping Federer, believes he can overcome the world number one.

"He's one of the few guys that I've probably played three or four really, really good matches against him and came up short," Roddick said of Federer.

"I think I have brought out the best in him a couple of times.

"All I can do is put my best foot forward, and hopefully right now he's thinking about it a little bit."