Naomi Osaka felt no pressure against Dominika Cibulkova in Pan Pacific Open in Tokyo

Japan's Naomi Osaka pulverised Slovak Dominika Cibulkova 6-2, 6-1 to reach the Pan Pacific Open quarter-finals on Wednesday in her first match since her historic US Open triumph.

Osaka, 20, who stunned Serena Williams to become Japan's first grand slam singles champion earlier this month, fired down 10 aces in a fearsome display of wham-bam tennis in Tokyo.

Third seed Osaka came out with guns blazing and ripped a fizzing forehand down the line on the first point, racing away to close out the first set with a pair of thundering aces.

A dazed Cibulkova had no answer to Osaka's firepower as the world No 7 wrapped up a comfortable victory with a ferocious cross-court backhand after just 59 minutes.

"I didn't really feel any pressure," said Osaka, who reached the Tokyo final in 2016. "It was really difficult because she's such a great player but I just thought it was more fun.

"My serve was really good," Japan's latest sporting celebrity added. "I'm just grateful to be able to play here again."

Cibulkova, an Australian Open finalist in 2014, conceded she had been thoroughly outplayed.

"It wasn't a match I was proud of," she said. "I couldn't find my rhythm. I felt I was in control of some rallies but it was too hectic."

Earlier, sixth seed Garbine Muguruza was upset by American qualifier Alison Riske 6-1, 6-2 in an abject performance from the Spanish former world No 1, who looked a shadow of the player who captured the 2016 French Open and 2017 Wimbledon titles.

Riske advances to face fourth seed Karolina Pliskova, another player to have previously held the women's top ranking.

The Czech recovered from a set down to beat Australia's Daria Gavrilova 4-6, 6-4, 6-4 and avenge her opponent's defeat of twin sister Kristyna earlier this week.

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But she made hard work of it, trashing a racquet in frustration as she slipped behind 4-1 in the deciding set before clawing her way back.

"The first set was in my hands but I made some stupid mistakes," said Pliskova, who ended Gavrilova's resistance with a clever drop volley after two hours and 21 minutes.

"I tried to stay positive and even though I broke my racquet I tried to keep fighting, because you always have a chance."

Second seed Caroline Garcia flirted with an early exit as the Frenchwoman overcame Russia's Anastasia Pavlyuchenkova 6-4, 2-6, 7-5 to reach the last eight of the prestigious Tokyo event, whose list of former champions includes Martina Navratilova, Steffi Graf, Kimiko Date and Martina Hingis.

Nadal out of China tournaments

Meanwhile, world No 1 Rafael Nadal pulled out of next month's China Open and Shanghai Masters tournaments on Wednesday, to allow his knee injury to recover.

The same injury forced the 32-year-old Spaniard to retire from his US Open semi-final against Juan Martin del Potro earlier this month.

He also missed Spain's Davis Cup semi-final defeat by France at the weekend.

The 17-time Grand Slam champion, who has suffered from tendonitis in his knee in the past, could return for the Paris Masters at the end of October.

"While the annoyances on my knee are nothing new, we have decided together with my medical and technical team, not to participate in the Asian tour to recover the knee the way we have always done," he tweeted.