Serena Williams lost in the first round of the Miami Open on Wednesday, still rusty in her return from pregnancy and unable to overcome a tough draw against Naomi Osaka, who won 6-3, 6-2.

The 20-year-old Osaka, who earned her first career title Sunday at Indian Wells, showed no signs of letup and overpowered the erratic Williams. Osaka had the stronger serve, and in rallies wore down Williams working her from side to side.

The matchup worthy of a final came about because both players are unseeded. Osaka is ranked a career-best 22nd, while Williams' ranking is 491st after her layoff of more than a year.

Another new mother and former No. 1, three-time Key Biscayne champion Victoria Azarenka, defeated Catherine Bellis 6-3, 6-0.

Williams has endured a first-round defeat only four other times, most recently at the 2012 French Open. The latest loss came at a tournament she has won a record eight times and considers her hometown event.

The match was the last for Williams at Key Biscayne, 90 miles south of her home in Palm Beach Gardens. The tournament is moving next year to the Miami Dolphins' stadium, and Williams helped with the ceremonial groundbreaking Monday.

After her defeat, she left without speaking to the media.

Because of Williams' ranking, she entered the draw as a wild card, and looked the part. She was a step slow to balls in the corners and often late with her swing, while she put barely half her first serves in play.

Osaka, who has both American and Japanese citizenship and lives in Fort Lauderdale, was playing her idol for the first time but didn't seem a bit fazed. She improved to 14-4 this year.

Things may get tougher for her in the second round, when she'll face No. 4-seeded Elina Svitolina.