Lauren Davis defeated Zhang Shuai of China 6-2, 6-3 at the Sony Open on Wednesday as former U.S. Open champion Juan Martin del Potro withdrew with a left wrist injury.
The 20-year-old Davis started 2014 ranked No. 72 and moved up to No. 55. From 1-1 in the first set, Davis jumped ahead to a 4-1 lead. In the second set, Zhang started with a 3-1 lead, but Davis won the next five games to take the match.
"I'm playing really well and I have a lot of confidence," Davis said. "I think there's a time for everything and I feel like this is my time."
Fellow American Christina McHale defeated Zheng Jie of China 6-4, 6-2.
Wildcard recipient Victoria Duval didn't fare as well as her fellow Americans, losing to Dutch qualifier Kiki Bertens 7-6 (5), 6-1.
Del Potro has been troubled by his left wrist since winning the Sydney title at the beginning of the year. At the recent Indian Wells tournament he played one doubles match, but then withdrew from the singles.
The 37th-ranked Andrea Petkovic of Germany, a top 10 player in 2011, posted a 6-3, 6-4 win over Maria-Teresa Torro-Flor of Spain.
In first round men's action, 80th-ranked Adrian Mannarino of France ousted 71st-ranked Nikolay Davydenko of Russia 6-4, 7-5.
Czech Radek Stepanek defeated Daniel Brands of Germany 6-3, 6-0.
Ivo Karlovic of Croatia claimed up a tough 7-6 (3), 7-6 (5) win over French qualifier Paul-Henri Matthieu.