Key Biscayne, Florida: Maria Sharapova is 0-5 in Key Biscayne finals. This year her loss came at the start.
Seeded No. 2, Sharapova was upset in her opening match Thursday night by wild-card Daria Gavrilova, 7-6 (4), 6-3.
The 21-year-old Gavrilova, who was born in Moscow and is based in Australia, earned her first victory over a top-10 player with a poised, aggressive effort before a near-capacity stadium-court crowd.
"I always believed, and that's probably why I won," she told the crowd. "I was very composed. I told myself to keep believing and keep trying."
Venus Williams began her 16th appearance at Key Biscayne by beating qualifier Urszula Radwanska 6-3, 6-2. Radwanska's sister, Agnieszka, won, as did former No. 1 Caroline Wozniacki.
Juan Martin del Potro, playing for the first time since Jan. 15, lost in the first round to Vasek Pospisil, 6-4, 7-6 (7). Del Potro had been sidelined with a lingering left wrist injury.
"I don't feel frustrated," del Potro said. "I have to take the positive things on my comeback. The score doesn't matter for now."
Sharapova lost serve four times, struggled with her return and found herself in a succession of long rallies, thanks to the 5-foot-5 Gavrilova's defensive skills. Sharapova blew an easy overhead on the next-to-last point of the first set, and then fell behind 3-love in the second set and could never pull even.
When she misfired on her final shot, Gavrilova did a pirouette and landed with a big grin. She had been 0-4 previously against top-10 players.
Ranked 97th, Gavrilova cracked the top 100 this week for the first time. She's making a comeback from a torn anterior cruciate ligament in her right knee in 2013.
Del Potro, who won the 2009 U.S. Open, had wrist surgery a year ago and played in just four tournaments in 2014. His ranking has fallen to No. 616, and his rustiness showed against Pospisil, to the disappointment of a partisan stadium crowd rooting for the Argentine.
Del Potro served at 5-4 in the second set and was broken. He double-faulted holding a set point, lost another set point and double-faulted again on the next-to-last point.
He converted only one of six break-point chances and hit just nine winners to 36 for Pospisil, a Canadian ranked 60th.
"I didn't serve well in the special moments," del Potro said. "But now I'm OK. Just playing the tournament and being here means a lot for me."
Del Potro said he felt fine physically after the match. He said his wrist is not yet 100 percent, but it's improving week by week.
"It doesn't matter how long it takes me to be at the top again," he said. "I just wanted to play tennis and without pain."
Williams, seeded 16th, lost serve three times and committed 21 unforced errors but still advanced in barely an hour. The typically windy weather on the island required an adjustment, she said.
"Those were really challenging conditions," she said. "The wind literally blew so many balls in that were going out."
Williams improved to 55-13 at Key Biscayne, where she won titles in 1998, 1999 and 2001.
"Been down here a little over forever," said Williams, who lives 90 minutes up I-95 in Palm Beach Gardens. "I love playing down here."
Aside from Williams, American women went 0-3, with Madison Brengle, Christina McHale and Varvara Lepchenko losing. Wozniacki, seeded No. 4, beat Brengle, 6-0, 6-1. No. 9 Andrea Petkovic swept McHale 6-2, 6-2. No. 28 Lepchenko was eliminated by Kaia Kanepi 6-2, 6-4.
No. 7 Agnieszka Radwanska defeated Anna Schmiedlova, 6-4, 7-5. Seeded players competed in their opening matches after receiving first-round byes.
On the men's side, Americans Steve Johnson and Ryan Harrison lost, as did former Australian Open runner-up Marcos Baghdatis. Martin Klizan won and will face No. 1 Novak Djokovic in the second round Saturday night.
Seven-time champion Serena Williams, Rafael Nadal and Andy Murray are among those playing their opening matches Friday.