Top-ranked Caroline Wozniacki won her opening match at the Sony Ericsson Open by beating Bethanie Mattek-Sands 6-2, 7-5 on Thursday.
Wozniacki, coming off her 14th career title last week at Indian Wells, erased eight of the 10 break points she faced. She hit only five winners but committed just 15 unforced errors to 49 for Mattek-Sands.
"Bethanie is a tough player to play," Wozniacki said. "You never know what to expect from her, and she's going for her shots no matter where she's standing on the court, and it's difficult to get the rhythm."
Seeded players were in action for the first time after having byes in the opening round. No. 16 Maria Sharapova, a two-time runner-up playing in the tournament for the first time since 2007, beat wild card Petra Martic 6-3, 6-2.
Among the seeds eliminated were No. 10 Shahar Peer, No. 13 Flavia Pennetta and No. 18 Nadia Petrova.
Peer was beaten by Anabel Medina Garrigues 6-2, 6-2. Pennetta lost to Iveta Benesova 6-3, 6-0. Petrova retired because of dizziness trailing Sabine Lisicki 4-6, 6-3, 3-1.
No. 4 Sam Stosur beat Zheng Jie of China 6-2, 6-1, and No. 6 Jelena Jankovic eliminated Monica Niculescu of Romania 6-3, 6-1. No. 11 Svetlana Kuznetsova, the 2006 champion, swept Simona Halep of Romania 6-0, 6-4.
In men's play, Kei Nishikori of Japan advanced to a second-round match against top-ranked Rafael Nadal on Saturday by beating Jeremy Chardy of France 7-6 (5), 6-2. Radek Stepanek of the Czech Republic will next play Roger Federer after beating Fabio Fognini of Italy 6-3, 7-6 (5). Pablo Cuevas of Uruguay will face 2010 champion Andy Roddick next after defeating Michael Berrer of Germany 6-4, 7-6 (2).
With Serena and Venus Williams missing the tournament because of injuries, Mattek-Sands was the highest-ranked American in the draw at No. 43.
The lone American woman left was Uzbekistan-born Varvara Lepchenko, who has lived in the United States since 2000 and became a citizen this year. The 82nd-ranked player has her second-round match on Friday.
Only seven Americans made the 96-player women's field, including two wild cards and two qualifiers - a reflection of the decline in U.S. tennis fortunes in recent years.