Williams hit 31 winners, broke serve 8-of-16 times and had her way at the net. Although she needed just 61 minutes to eliminate Bartoli, Williams maintained the match was tougher than it appeared.
"I think the score line wasn't true to the match at all," she contended. "She played really well. I definitely had to work hard to win that match. I knew I was going to, because I had seen her play before."
The 18-year-old Bartoli reached the quarterfinals of a Tier I event for the first time after facing a local wild-card entry, a lucky loser, a qualifier and an injured player in consecutive matches.
Playing against the defending champion was another thing. Although Bartoli felt like she could handle Williams' groundstrokes, she was unable to deal with the power of the overall package.
"When she hit in the baseline, her ball is not too quick," Bartoli said in broken English. "I feel I can play every time, and I feel I can run her. When she runs, she's definitely not very good, so ...
"When the rally beginning, I was quite confident in my shot. But when I serve, and she come to the court, I say, 'Oh, my God. Oh, my God, what going to be happening?' I was going just to serve and, boom, (she would) return winners. I serve, (she would) return winners."
Williams has dominated since her triumph here last season. She has won four straight Grand Slams and five other events in the last 12 months.
Other than her sister Venus, one of the few players to bother Williams has been Kim Clijsters, a possible semifinal foe here. The third-seeded Belgian squared off against No. 9 Jelena Dokic of Yugoslavia on Tuesday night.
Clijsters defeated Serena Williams in the final of last year's season-ending tournament in Los Angeles, but squandered a 5-1 lead in the final set of the Australian Open in January.
In another evening match, men's second seed Andre Agassi of the United States encounterd Australian Mark Philippoussis. Agassi was seeking his third straight title here and sixth Miami crown overall.
Marcelo Rios, whose promising career has been stunted by injuries, pulled out of this event for the second straight year. The 28th seed from Chile was to have faced No. 13 Paradorn Srichaphan of Thailand in the fourth round but could not play due to a back injury.
After reporting pain to trainers following his third-round win over third seed Juan Carlos Ferrero, the 1998 champion underwent an MRI on Monday. The test showed he had a slight herniation of the fourth and fifth vertebrae in his back.
Last year, Rios split the first two sets of his semifinal against Andre Agassi before retiring with a thigh injury.
Srichaphan next meets Todd Martin in the quarterfinals. The 32-year-old American, who ousted No. 6 Andy Roddick in the third round, dispatched unseeded Czech Radek Stepanek, 6-3, 6-2.
In the same half, fourth seed Carlos Moya of Spain defeated Nicolas Escude of France, 6-4, 7-6 (10-8), to set up a meeting with surprise quarterfinalist Robby Ginepri.
Ginepri, a wild card from the United States, has been enjoying the best three weeks of his career, overcoming former U.S. Open champion Marat Safin in Indian Wells two weeks ago, and seeds Alex Corretja and James Blake at this event.
The 20-year-old reached his second quarterfinal in a row in a Tennis Masters Series by beating Korean Hyung-Taik Lee, 6-3, 6-2.
Fourth seed Roger Federer of Switzerland continued his strong form with a 6-3, 6-2 victory over No. 14 Sjeng Schalken of the Netherlands.