Johanna Konta has been knocked out of the Miami Open
Defending champion Konta, ranked 14th, won the first set but eventually lost 5-7 6-1 6-2.
The 26-year-old had beaten world number eight Williams in straight sets in the semi-finals 12 months ago.
Williams, 37, will play Monica Puig of Puerto Rico or fellow American Danielle Collins in the quarter-finals.
Konta saved set point to turn the first set - which lasted just over an hour - in her favour and take the early advantage.
But Williams, a 10-time Grand Slam singles champion, cruised through the second set to even the score, with Konta requiring a medical timeout before the deciding set.
The Briton hit three aces in a row to save a break point in the third set, but Williams continued her dominant comeback to serve for the match