Miami Open: Naomi Osaka, Ashleigh Barty reach quarter-finals; Stefanos Tsitsipas into last 16 after beating Kei Nishikori

Reigning US and Australian Open champion Naomi Osaka advanced to the Miami Open quarter-finals on Monday, stretching her win streak to 23 matches by defeating Belgian 16th seed Elise Mertens 6-3, 6-3.

The second-ranked Japanese star will next face Greek 23rd seed Maria Sakkari, who outlasted US 29th seed Jessica Pegula 6-4, 2-6, 7-6 (8/6) at the WTA and ATP Masters Series event.

"I started off really well and then for some reason I became a little sluggish. My speed didn't really feel the same," Osaka said.

"I thought it was really mental from then on. In the end, it was who fought the hardest."

Four-time Grand Slam champion Osaka broke at love for a 2-0 lead and broke again for a 5-1 edge but Mertens broke Osaka at love in the seventh game and denied four set points for Osaka to hold in an eighth game that lasted almost 10 minutes.

Osaka, seeking her first Miami crown, saved a break point with her third ace and finally finished off the first set on her seventh chance with a service winner.

In the second set, Osaka needed her fifth chance of the third game to break Mertens, only for the Belgian to quickly break back to 2-2 and hold to 3-2 before requiring treatment for a right shoulder injury.

Osaka won the last four games after that, breaking at love in the seventh game, holding at love in the eighth and breaking again to end matters after 88 minutes when Mertens netted a forehand.

Top-ranked defending champion Ashleigh Barty battled into the quarter-finals by outlasting former world number one Victoria Azarenka 6-1, 1-6, 6-2.

Australia's Barty broke five times, the last when Azarenka netted a forehand volley to end matters after one hour and 52 minutes at Hard Rock Stadium.

Barty, who had to save a match point on the way to a three-set victory over Kristina Kucova in the second round, plays Tuesday against seventh seed Aryna Sabalenka of Belarus, who downed Czech 19th seed Marketa Vondrousova 6-1, 6-2.

"I'm just excited I get to be in another quarter-final of a big event," Barty said.

"I still feel like there's a lot better tennis left out there for me. I still don't feel like I'm playing my very best just yet, but I'm fighting through and I'm finding ways to win, which is great."

The 2019 French Open winner dominated the first set but Azarenka broke for a 2-0 lead in the second, then fought back from 0-40 down to hold for 3-0 on the way to forcing a third set.

After an early exchange of breaks, Azarenka denied Barty on two break chances in the fourth game but the 24-year-old Queenslander broke in the sixth game and again on the last point.

"First two sets probably had some pretty big momentum swings," Barty said. "But I think the third set was a real tussle."

Ukraine's fifth-seeded Elina Svitolina rallied to defeat Czech ninth seed Petra Kvitova 2-6, 7-5, 7-5.

"I'm really happy with how I bounced back from being a set down," Svitolina said. "I'm happy with how I'm playing and handling the pressure. I had to bring my best game to win."

Svitolina's quarter-final foe will be Latvia's 57th-ranked Anastasija Sevastova, who eliminated Croatia's 338th-ranked Ana Konjuh 6-1, 7-5.

Tsitsipas, Rublev win

On the men's side, Greek second seed Stefanos Tsitsipas defeated Japanese 28th seed Kei Nishikori 6-3, 3-6, 6-1 after an hour and 56 minutes to reach the last 16.

Tsitsipas will next face Italian 24th seed Lorenzo Sonego, who dispatched Colombian Daniel Galan 7-6 (8/6), 6-3.

Russian fourth seed Andrey Rublev routed Hungarian 29th seed Marton Fucsovics 6-2, 6-1, to reach the fourth round.

"I've played some great tennis last couple of months," Rublev said. "We'll see if I can keep going."

The 23-year-old from Moscow will next face Croatia's 32-year-old Marin Cilic, the world number 45 who beat Italy's 19-year-old Lorenzo Musetti 6-3, 6-4.

"Very pleased," Cilic said. "Just have to battle it out with these youngsters. I've got to keep the heads high for the veterans on the tour."

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