Defending champion Naomi Osaka safely advanced to the third round, but Kei Nishikori was sent packing as the two players from Japan completed their opening matches at the Miami Open on Friday.

In women's action, World No. 1 and top seed Osaka defeated Belgian qualifier Yanina Wickmayer 6-0, 6-7(3), 6-1 to book herself a third-round clash with 27th seed Hsieh Su-wei of Taiwan. Hsieh beat American Alison Riske 6-2, 7-5.

"Of course I'm really happy that I was able to win the match," said Osaka, who hit 14 aces against the 141st-ranked player but also threw her racket in a fit of pique when Wickmayer forced the second set tiebreak.

"It was really hard for me, emotionally, in the second set because I just started thinking about winning. It's normal. It's the first match I have played here. I wasn't able to defend in Indian Wells, so of course I want to do really well here," she said.

Osaka admitted she had difficulty controlling her emotions during the 2-hour, 9-minute match, saying she feels bad for her new coach Jermaine Jenkins who has been stressing that she must trust herself and remain calm.

"I feel sorry for him during this match because I was throwing my racket and stuff, and that's completely against what he was saying. But I felt really frustrated. At least I didn't crack it, though."

On the men's side, Nishikori, the fifth seed, was stunned by 44th-ranked Dusan Lajovic 2-6, 6-2, 6-3, giving the Serbian his third career win over a top-10 player.

Nishikori struggled with his serve from the second stanza, allowing Lajovic to capitalize to break five times over the last two sets to snatch the win in just under two hours.

"He started raising his game in the second and third sets. I didn't play terribly, but it wasn't my best tennis either," said Nishikori, who made a third-round exit from the first stop of the March Masters, the BNP Paribas Open, earlier this month.

Osaka crashed out in the fourth round of the same tournament in Indian Wells.

Elsewhere at the Miami Open, Japan's Misaki Doi was knocked out by Polona Hercog of Slovenia 7-6(4), 6-4 in the second round of the women's singles.

In the first round of the women's doubles, the all-Japan team of Miyu Kato and Makoto Ninomiya were eliminated by the Japan-Belarus pair of Shuko Aoyama and Lidziya Marozava, 6-3, 6-4.

Japan's Ben McLachlan and Britain's Neal Skupski suffered a first-round defeat in the men's doubles.