NAOMI Osaka beat Angelique Kerber 6-2 6-3 on Thursday in a one-sided clash of two former world number ones to reach the third round of the Miami Open, where Garbine Muguruza and Simona Halep withdrew with injuries.

Four-times Grand Slam champion Osaka made a strong start against German 13th seed Kerber, grabbing an early break to go 2-0 up and dominating on serve in a 29-minute opening set.

Osaka denied Kerber any chance of getting back into the match and had total control of her game as she went on to close out the 60-minute contest on her third match point when her opponent sent a forehand long.

The victory was Osaka's first against Kerber since the 2017 U.S. Open and denied her opponent a fifth consecutive win in their head-to-head series, which the German now leads 4-2.

It also marked Osaka's first win over a top-20 player since last year's Miami Open.

"It is a confidence booster, but I didn't go into the match not confident, if that makes sense," Osaka told reporters. "For me, it kind of just shows all the hard work that I have been doing throughout the year."

Up next for Osaka, who arrived in Miami following a second-round loss at Indian Wells where she was reduced to tears after being heckled during the match, will be Czech Karolina Muchova, who beat U.S. Open finalist Leylah Fernandez 6-4 7-6(3).

In other second-round action, Olympic doubles champion Katerina Siniakova was down a set and a break but rallied to secure a 3-6 6-4 7-5 win over U.S. Open champion Emma Raducanu in a match lasting nearly three hours.

It was a remarkable turnaround for the unseeded Czech, who made several calls for the trainer, the first while she was trailing 4-1 in the first set and appeared to be labouring in the heat.

"For me it was really tough. I was more fighting with myself in the opening and then I kind of got some relief and it was much better," said Siniakova. "I just felt better as the match was going on"

Also reaching the third round were Tunisian eighth seed Ons Jabeur, American ninth seed Danielle Collins, Briton Heather Watson, Alison Riske and Aliaksandra Sasnovich.

Spanish seventh seed Muguruza, who was supposed to face qualifier Lauren Davis on Friday, withdrew with a shoulder injury and said she hopes to be ready for claycourt action in Madrid in late April.

Romanian 21st seed Halep also withdrew ahead of her second-round match against Australian wildcard Daria Saville due to a leg injury the former world number one said would keep her out of action for three weeks.

On the men's side, Briton Andy Murray looked sharp in his 7-6(4) 6-1 win over Federico Delbonis. The former world number one held his nerve through a tight first set before breaking serve to open the second and pulling away.

Murray, who has won the tournament twice, will face top seed Daniil Medvedev of Russia in the second round.

Other first round winners included Americans Tommy Paul, Sebastian Korda and Jenson Brooksby, Australian Jordan Thompson and Japan's Yoshihito Nishioka.