Rafael Nadal withdrew before his opening match, saying he needs more time to recover from a hip injury. The second-ranked Spanish star hasn't played since retiring in the Australian Open quarterfinals.
Stephens saved a match point in the second-set tiebreaker against the left-hander. The American won the U.S. Open last year for her first Grand Slam title and took the Acapulco hardcourt event in 2016. She'll face Switzerland's Stefanie Voegele, a 6-3, 6-4 winner over Greece's Maria Sakkari.
Second-seeded Kristina Mladenovic also overcame a slow start to advance, beating Brazil's Beatriz Haddad Maia 3-6, 6-3, 7-5. Mladenovic will play seventh-seeded defending champion Lesia Tsurenko of Ukraine, a 6-3, 6-1 winner over Arina Rodionova of Australia.
Third-seeded Daria Gavrilova of Australia beat Mexico's Renata Zarazua 7-5, 6-3 to set up a match against Veronica Cepede Royg of Paraguay. Royg upset fifth-seeded Irina-Camelia Begu of Romania 7-6 (3), 6-1.
Fourth-seeded Zhang Shuai of China beat Jil Teichmann of Switzerland 7-6 (5), 6-4.
In men's play, second-seeded Alexander Zverev beat fellow German Peter Gojowczyk 7-6 (6), 6-3. Zverev will face Ryan Harrison of the United States. Harrison edged Argentina's Diego Schwartzman 6-3, 5-7, 6-4.
Third-seeded Dominic Thiem of Austria topped Denis Shapovalov of Canada 6-2, 6-3. The Argentina Open winner two weeks ago on clay, Thiem will face sixth-seeded Juan Martin del Potro of Argentina, who beat David Ferrer of Spain 6-4, 4-6, 6-3.