Japan's Naomi Osaka followed her first career title by knocking former world No. 1 Serena Williams out of the Miami Open in straight sets Wednesday.

Fresh from winning Sunday's BNP Paribas Open final, the 20-year-old Osaka did not concede a break point on her way to beating childhood idol Williams 6-3, 6-2 in the opening round.

On the comeback trail following the birth of her first child, Williams looked some way off regaining the form that earned her 72 WTA singles titles, including eight in Miami.

She hit 28 unforced errors to just 16 winners as she exited the tournament before the fourth round for the first time, according to the WTA website. Osaka, meanwhile, hit 18 winners and seven aces as she rode her powerful first serve to victory in 1 hour, 17 minutes.

Miami Open tournament director James Blake earlier in the week urged a rethink of the WTA tour's seeding rules that paired Williams with rising star Osaka in the opening round after she dropped to 491 in the rankings during her 13-month maternity absence.

Osaka, who jumped 22 places to No. 22 following her victory at Indian Wells, spoke of her admiration for Williams after the match, saying she "wanted to impress" the player who inspired her to pursue a tennis career.

"I was extremely nervous coming on to the court. I don't know if anybody knows this, but Serena is my favorite player," Osaka said in her post-match interview on court. "Just playing against her is kind of like a dream for me, so I'm very grateful that I was able to play her, and it's even better that I was able to win."

In her press conference, Osaka admitted still feeling star-struck when Williams congratulated her on the win.

"I kind of blanked out. I'm pretty sure she said, 'good job'," Osaka said. "It's weird when you grow up watching someone and wanting to be exactly like them, and then you have the chance to play them."

"She's the main reason why I started playing tennis. I've seen her on TV so many times and I've always been cheering for her. So for me to play against her and just sort of trying to detach myself a little bit from thinking that I'm playing against her, and just try to think I'm playing against just a regular opponent, was a little bit hard for me."

Osaka will play the fourth seed, Ukrainian Elina Svitolina, in the second round. The two have split their four career meetings.

In other first-round results, Japan's Yuichi Sugita was knocked out by Robin Haase of the Netherlands 6-4, 3-6, 6-1. The Dutchman served eight aces to Sugita's one as he wrapped up their first meeting in one hour, 51 minutes.

World No. 43 Sugita, who climbed in the rankings last year after starting outside the top 100, said he felt the effects of a busy schedule as he faded in the third set after fighting back to win the second.

"I lost because I couldn't raise the level of my play in the third set," Sugita said. "I feel as though my energy level has dropped."