Still rusty in her return from pregnancy, Williams was unable to overcome a tough opening draw at the Miami Open and lost Wednesday to Naomi Osaka, 6-3, 6-2.
On the final point, Williams thundered forward to attack an easy shot at eye level, whacked the ball six feet past the baseline and responded with a sheepish smile.
Not that motherhood has mellowed Williams. She left without talking to the media.
The 20-year-old Osaka, who earned her first career title Sunday at Indian Wells, said she was nervous at the start playing her idol for the first time. Osaka said her goal was to avoid Williams winning 6-0, 6-0.
"She's the main reason I started playing tennis," Osaka said. "I just wanted her after the match to know who I am."
Mission accomplished. What did Williams say when they shook hands after the match?
"She said 'good job' and stuff," Osaka said. "I kind of blanked out, but I'm pretty good she said 'good job.'"
Osaka, who has both American and Japanese citizenship and lives in Fort Lauderdale, raised her profile with last week's title run and showed no signs of letup and overpowered the erratic Williams. Osaka had the stronger serve and wore down Williams in rallies working her from side to side.
The matchup worthy of a final came about because both players were unseeded. Osaka is ranked a career-best 22nd, while Williams' ranking is 491st after a layoff of more than a year.
Another new mother and former No. 1, three-time champion Victoria Azarenka, defeated Catherine Bellis 6-3, 6-0. Seeded players are in action beginning Thursday.
Williams has been eliminated in the first round only four other times, most recently at the 2012 French Open. The latest defeat 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, where Williams helped with the ceremonial groundbreaking Monday.
Because of Williams' ranking, she entered the draw as a wild card, and looked the part. She needed only eight shots to win the match's first five points, but things became much more difficult after that.
Playing her fourth match since returning to the tour, Williams 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. Early in the second set she resorted to her patented scream of "Come on!", but the outburst failed to turn things around.
Osaka was heartened that she provoked Williams to shout at herself.
"Sometimes she plays matches where she doesn't say 'Come on!' at all," Osaka said. "That's a little bit sad, because it's like, 'Do you think she's trying?' So I just wanted her say 'Come on!' once. I knew then maybe she would be trying a little bit."
Williams tried, but typical of her struggle was an exchange midway through the last set. She scrambled forward to scoop a ball back, retreated to hit an awkward, lunging overhead, and then stood and watched helplessly as a crosscourt forehand from Osaka whizzed past for a winner.
Osaka's serve reached 116 mph, and she was never broken. When Williams pushed her to deuce three times in one game, Osaka closed it out with consecutive aces, and said she drew inspiration from her hero.
"Sometimes when I'm in a really hard position when I'm serving, I'm like, 'What would Serena do?'" Osaka said.
That approach has served Osaka well. She's 14-4 this year and will next face No. 4-seeded Elina Svitolina as the tournament's new player to beat.
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