Naomi Osaka stunned the returning Serena Williams in the first round at the Miami Open on Wednesday, adding the 23-time grand slam winner to her growing list of scalps.

Williams was playing only her second tournament since giving birth to her first child having lost to sister Venus at Indian Wells last week.

Osaka won that tournament to collect her first singles title and the Japanese showed further superb form, a 6-3 6-2 victory over Williams following a straight-sets triumph against another big name in world number one Simona Halep in the last four of the Indian Wells Masters.

Showing some initial nerves, Osaka had to battle to hold her first service game, although Williams was displaying understandably rusty movement as her comeback continues.

But the 20-year-old drew first blood, piling pressure on the Williams serve and forcing the American to chip a backhand wide to concede the first break of the match.

And Osaka maintained her momentum by grabbing the first set with another break, Williams unable to cope with the Japanese's powerful and accurate groundstrokes.

Williams had to save break point in her first service game of the second set, a thumping serve down the middle helping her to hold.

The flawless Osaka would not be denied, though, moving 3-1 up by breaking Williams again, and the biggest win of her career was soon wrapped up.

Williams, an eight-time Miami Open champion, saved the first match point but sent a routine forehand long to give rising star Osaka further glory.

Despite knocking out Williams the draw does not get much easier for Osaka, who will face fourth seed Elina Svitolina in round two.