Struggling former world number one Garbine Muguruza said she was "not scared" of her deepening slump after losing in the quarter-finals of the Pan Pacific Open in Tokyo on Friday.

The number three seed lost 6-4, 6-2 to Russia's Liudmila Samsonova to continue a dismal run of results this year for the two-time Grand Slam champion.

Muguruza has not managed to string together more than two wins at a tournament and fell out of the world top 10 earlier this month.

But she said she had "already recovered" from her latest defeat and was looking forward to her next tournament.

"I don't think I have to be too sad about it -- I played good, she played better, and that's pretty much it," said the Spaniard, now ranked number 12 in the world.

"I don't want to overthink, I want to keep going. I've had worse losses where I didn't think that I played good but this one I felt like I was there."

Muguruza, who won the French Open in 2016 and Wimbledon a year later, started the year ranked number three but has since lost in the first round of six tournaments.

She said she "definitely felt at the beginning that this year was going to be a tough one", but was "not scared of a tough year whatsoever".

"I'm not saying it's easy but I also have the experience of not having to be too sad about it," she said.

"Because I've been through many ups and downs, I have a tough wall where this is not going to bother me."

Muguruza was the latest big name to fall at the Pan Pacific Open.

Top seed Paula Badosa, US Open semi-finalist Caroline Garcia and Wimbledon champion Elena Rybakina all made early exits, while Naomi Osaka withdrew in the second round with abdominal pain.

Samsonova will play Zhang Shuai in Saturday's semi-final after the Chinese veteran beat Croatia's Petra Martic 7-5, 6-2.

Zhang saved a match point before ousting Garcia in the previous round, and she said she had to again dig deep to beat the in-form Martic, who had won 15 of her previous 20 matches.

"Today was not easy for me because her tennis is really beautiful," said Zhang.

"She has everything, so for me it was a challenge mentally, not only technically."

Fast-rising Chinese teenager Zheng Qinwen also advanced to the semi-finals with a 6-4, 7-5 win over American Claire Liu.

The 19-year-old Zheng upset Badosa in the second round and will next face Russian number four seed Veronika Kudermetova, who beat Brazilian number five seed Beatriz Haddad Maia 6-7 (4/7), 7-6 (8/6), 6-1.

"I had to give everything I had to win this match," said Zheng.

"I'm so happy that I could finish the match in two sets because the second set was very difficult. I'm happy with my performance today."