Williams, the top seed, rallied past the sixth-seeded Capriati, 4-6, 6-4, 6-1, in just over two hours at the $6.21 million hardcourt tournament. She captured her 22nd career title and collected $393,000.
"I haven't been on my game today," said Williams, who has won the last four Grand Slams. "But I'm going to go home and I'm not going to take some time off. I'm actually going to practice because I don't think I've been doing enough practicing.
"That's why my form is off. I've been hanging out, relaxing, not working. I can't keep doing it like that. There's people out there who's working hard and coming closer."
The contest was a rematch of last year's final, which Williams won in straight sets. The only other time in the tournament's 19-year history that a repeat final occurred was in 1987-88, when Steffi Graf twice beat Chris Evert.
Williams improved to 17-0 this season and 8-4 lifetime against Capriati. Eight of those matches have gone three sets and Williams has won the last seven encounters.
"I don't think so," Williams said when asked if she could go undefeated this year. "It's gonna be really hard. It's impossible to win them all. If you could, I would love to. That's going to be what I'm going to try to do, but I don't know if it's possible."
On the men's side, second seed and two-time defending champion Andre Agassi beat ninth-seeded Albert Costa of Spain, 6-2, 6-4, to move within one win of his third straight title here and sixth career championship in Miami. He has not lost here since losing the 2000 semifinals to Brazilian Gustavo Kuerten.
If he wins Sunday against Spaniard Carlos Moya, Agassi would become the all-time leader in titles here, one more than his wife Steffi Graf.
"It would be amazing," he said. "It would be nice because then I can have Steff bring all her trophies home from this tournament. As of right now, she's not allowed to have all of them there. If I win six, I'll let her bring home the five that she won."
The 32-year-old American, who improved to 4-1 lifetime against Costa, is looking for his third tournament victory of the season. He captured his eighth career major at the Australian Open in January and won at San Jose in February. His only defeat of the year was to Thomas Enqvist in the first round of Scottsdale last month.
His match was initially scheduled for Friday, but was delayed one day when Thursday's quarterfinals matches were suspended by rain.