Reigning champions and Serena Williams and Andre Agassi both advanced in straight sets at a rainy Nasdaq-100 Open in Key Biscayne.
Agassi, the three-time defending champion and fourth seed, routed 28th seed Max Mirnyi of Belarus 6-2 6-2 in just 52 minutes for his 19th consecutive match win in Key Biscayne.
"It feels good," said Agassi, who next plays Argentine Agustin Calleri. "Days like this are few and far between, so I definitely feel great about the performance.
Second seed Andy Roddick of the United States and fifth seed Carlos Moya of Spain both had tougher routes to the quarter-finals.
Roddick overcame 26th seed Jonas Bjorkman of Sweden 6-4 3-6 6-3 in a preview of next week's Davis Cup quarter-final tie, and Moya knocked out 27th seed Dominik Hrbaty of Slovakia 7-5 1-6 6-3.
In the fourth round of the womens' competition Serena Williams continued her comeback from an eight-month injury nightmare by beating 17th seed Maria Sharapova of Russia 6-4 6-3 in a match delayed two hours by rain.
She was stretched to three sets by another Russian, Elena Likhovtseva, in the third round on Sunday.
"I was a bit sloppy, but confidence-wise I feel like I'm getting better with every match," the six-time Grand Slam champion said.
Serena, who won her 15th straight match at this event and continued her quest for a third straight Nasdaq title, now meets American Jill Craybas, a 4-6 6-4 7-6 (7/2) winner over ninth seed Paola Suarez of Argentina.
Not to be outdone by her younger sister, Venus, the second seed, dispatched 15th seed Francesca Schiavone of Italy 6-1 6-3.
She missed the last five months of last season with an abdominal strain.
Before arriving in Miami, 23-year-old Venus had only won two matches in a row once this season. Now she has put together three as she moves toward her third title here.
Venus now faces Elena Dementieva after the fifth-seeded Russian rallied to defeat 12th seed Jelena Dokic of Yugoslavia 5-7 6-1 6-3.
Another Russian, eighth seed Nadia Petrova, crushed 24th seed Alicia Molik of Australia 6-2 6-1 but her fellow countrywoman - 10th seed 10 Svetlana Kuznetsova, fell to 29th seed Karolina Sprem of Croatia 3-6 7-5 6-4.