The fifth-seeded Ivanovic won her first-round match in the Bank of the West Classic, defeating Sabine Lisicki 7-6 (2), 6-1 on Tuesday. The former world No. 1 lost to Lisicki in three sets in their previous meeting at Wimbledon.
“My mentality has changed,” Ivanovic said. “Sometimes just a few points make a difference and it helps to be positive in a tight situation. I did that today in the tiebreak.”
She also hired Dejan Petrovic and was playing her first tournament with him as her coach.
Williams, also a former No. 1, beat qualifier Paula Kania 6-3, 6-2. She’ll next play fourth-seeded Victoria Azarenka, whom she has beaten in three previous meetings.
“She’s always a tough competitor,” Williams said.
Ivanovic has won three tournament titles on the WTA Tour this season after failing to win a tournament in 2013. She owns 14 career titles.
Ivanovic fell behind in the first set and rallied to win it in a tiebreaker.
The Serbian next faces qualifier Carol Zhao, a sophomore at Stanford who was leading Yanina Wickmayer 6-2, 1-0 when the Belgian retired because of a viral infection.
Williams, who made her pro debut in the event 20 years ago, has won two titles and reached the finals five times in 12 previous appearances.
Ranked 25th and no longer the dominant figure on the tour, Williams thinks she still has a Slam title in her.
In other matches, Garbine Muguruza downed defending tournament champion and sixth-seeded Dominika Cibulkova, 6-2, 4-6, 6-2, Monica Puig beat seventh-seeded Carla Suarez Navarro, 6-3, 7-5 and Daniela Hantuchova topped Paula Ormaechea, 6-3, 6-4.