Osaka, who won her first big title in Indian Wells last weekend, was superior throughout and fully deserved her 6-3, 6-2 win in just one hour and 17 minutes
Nervous Naomi Osaka thrilled to beat hero Serena Williams at Miami Masters tennis event
Serena Williams crashed out of the Miami Open in the first round here Wednesday, slumping to a straight sets defeat against Japan's Naomi Osaka in the latest setback to the former world No 1's comeback.
Osaka, who won her first big title in Indian Wells last weekend, was superior throughout and fully deserved her 6-3, 6-2 win in just one hour and 17 minutes.
It was Williams's fourth match and second tournament appearance since returning from 13 months of maternity leave - yet the American's movement and inability to stay in the rallies suggested her comeback could be more arduous than expected.
The defeat was the first time Williams has lost her first match at an American hard court championship for 21 years.
Williams, 36, may have been unfairly unseeded here due to her lack of action over the past year, but could have no complaints with the defeat.
In her comeback in Indian Wells, she made it through to Round 3, where she fell to sister Venus in straight sets.
Taking on Osaka was always going to prove a tough task - and so it did.
Williams refused to speak to the media afterwards. The WTA and tournament will decide if she will be fined, with a potential cost expected to be about US$1,000 (Dh3,670).
She released a brief statement more than 90 minutes after the end of the match that read: "Every tournament is an opportunity for me to better understand the areas I need to improve to be my best.
"Naomi played a great match and I look forward to continuing my return by progressing every day.
"I am so grateful for my fans who continue to support me every step of this incredible journey."
Osaka, though, was understandably delighted.
"I was nervous but once the match started I was OK," said the Japanese rising star, who next faces Ukrainian Elina Svitolina, the world No 4, in the second round.
"Serena is the main reason why I started playing. I have seen her on TV so many times, so for me to be playing against her, trying to detach myself from the fact I was playing Serena, was hard. It took me three games [to get her head around it].
"It was a different type of nerves. I have never been like that against a specific person. The final in Indian Wells was different because I had never been in that position."
Williams looked a shadow of her former self, yet her opponent was too focused on her own game to notice any glaring deficiencies in the American's game.
"I wasn't paying attention if she was struggling or not," Osaka said. "I would freak myself out if I was looking too much at her. She did hit a lot of shots which almost made me fall over and I was like 'Wow, that's a Serena shot'."
Williams, an eight-time champion in Key Biscayne, started strongly and held her serve to love but it took a while for the American to find her range.
Osaka, 20, showed some understandable nerves but was able to stay in the game and when she broke in the seventh game, it was the perfect chance for the Japanese player to take control.
The world No 22's serve - which was being clocked at an impressive 115 mph - as well as the power in her ground strokes allowed her to stretch the lead to 5-3.
Osaka was hitting the ball beautifully and broke again to take the first set with relative ease.
Osaka broke once more to make it 3-1 in the second and she never looked back, sealing the win when Williams hit an easy passing shot long.
"I think she was sad that it ended like that " Osaka said. "I was new to the tour when she left after the Australian Open, so to shake her hand at the end was pretty cool. She said, 'Good job,' but I kind of blanked out.
"It's weird playing against someone who you've grown up watching. There's a respect thing but you also want to win really bad. I just wanted her to know who I am."