The Spaniard beat Greek qualifier Despina Papamichail 6-4, 6-2 to move into the quarter-finals in Tokyo and gain “a little bit more confidence” in a poor 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.
At the recent US Open she squandered two match points before losing to Petra Kvitova in the last 32.
Muguruza gave Wednesday’s win a cautious welcome but said it was far too early to know whether she is turning a corner.
“It’s been a tough season, a little bit of a roller coaster,” said the 28-year-old, who is now ranked 12 in the world.
“I don’t know if I’m going to be able, with these last tournaments, to have a different feeling. That’s my goal.”
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 struggled to put away Papamichail when she had her on the ropes several times in a tense first set and admitted that she still lacked the killer instinct.
“I think that it’s all about confidence and how well you’re playing or how many matches you’ve been playing,” she said.
“Definitely this year I feel that I haven’t played that many matches so obviously when the important moments come, you really feel that confidence that is not quite there yet.”
Muguruza knows she must stay patient.
“I think also trying and trying and trying, at some point I will get rewarded,” she said.
Zhang Shuai beats second-seeded Caroline Garcia
Zhang Shuai upset second-seed Caroline Garcia 4-6, 7-6 (5), 7-6 (5) in the second round of the Pan Pacific Open on Wednesday.
Garcia broke Zhang’s serve in the first game of the first set and pressured the world No. 28 with strong serves and powerful ground strokes, winning the set with her 10th ace.
But despite serving 27 aces in the match, Garcia, the U.S. Open semifinalist, couldn’t maintain her form and Zhang used her speed and precision to take the upper hand in the second set.
Zhang, 33, said her many years of playing and training in Japan paid off.
“I was trying to stay calm. I told myself in the tough moments how I never lose on this court so today I will win,” she said. “This is my home tournament so that gave me a lot of motivation. Everything is positive here and that’s why I played really well.”
(with inputs from AP)