Stephens defeated Stefanie Voegele 6-3, 6-2 in first-day action at the Premier-level event that carries a total purse of $2.3 million.
Stephens lost her serve just once and won two-thirds of the points on serve in finishing off the match in 59 minutes. She had four breaks -- two in each set.
Venus Williams also advanced, taking out Mona Barthel 6-3, 6-1, winning 60 percent of the points in the match. That includes 57 percent off Barthel's serve. Williams, who is unseeded, finished with six breaks.
Seeded players went 5-0 Sunday. In addition to Stephens' win, 12th-seeded Samantha Stosur topped Alize Cornet 2-6, 6-4, 6-1 with three final-set breaks and No. 14-seed Kirsten Flipkens came away a 7-6 (7-4), 7-5 winner over Barbora Zahlavova Strycova. Also, 15th-seeded Sorana Cirstea beat Julia Goerges 6-4, 6-4 and No. 16 Dominika Cibulkova topped Urszula Radwanska 6-3, 6-3.
Unseeded winners, along with Williams, Sunday included Svetlana Kuznetsova, Belinda Bencic, Lucie Safarova, Maria-Teresa Torro-Flor and Elina Svitolina.