Serena Williams capitalised on a critical double-fault by Yaroslava Shvedova to survive a 69-minute first set and win her opening match in Miami 7-6 (7), 6-2.
The American had to erase a 5-3 deficit in the first set, and then fell behind 6-3 in the tiebreaker.
Shvedova pushed a forehand into the net, then the Kazakh hit a nervous double-fault that allowed Williams to exhale.
She won the next two points with aces, then closed out the set with a backhand winner.
Her game steadied in the second set, when she had 18 winners and only 10 unforced errors - bad news perhaps for opponents to come in the tournament Williams considers her hometown event.
The top-ranked Williams is aiming for a record seventh Key Biscayne title, and her second in a row.
Fourth-seed Maria Sharapova, a five-time runner-up in the tournament, joined Williams in the third round by beating Kurumi Nara 6-3, 6-4.
Hewitt reaches landmark
In the men's draw, former world number one Lleyton Hewitt became the third active man to win 600 matches when he rallied past Robin Haase of the Netherlands 3-6, 6-3, 6-3.
Fellow Australian Bernard Tomic lasted only 28 minutes in the shortest match since the ATP started keeping such records in 1991, losing to Jarkko Nieminen 6-0, 6-1.
Tomic, who won just 13 points, is mounting a comeback from surgery on both hips and was playing for the first time since January.
The 33-year-old Hewitt also staged a comeback as his win put him equal with Roger Federer (942 wins) and Rafael Nadal (675) in reaching the 600 match milestone.
"Not many people get the opportunity to get close to that, so it means I have been around for an awfully long time," Hewitt said.