Madison Keys maintained her 100 per cent record against Daria Kasatkina by beating the Russian in the first round of the Pan Pacific Open.

Keys had dropped just one set in five previous meetings and rarely looked troubled in extending that record with a 6-3 6-4 victory in 75 minutes in Osaka.

The fifth-seeded American's aggressive style, which led to her hitting 24 winners, proved decisive as she set up a second-round clash with qualifier Zarina Diyas.

"I feel good, I'm always a little bit nervous ahead of my first matches," Keys said. "It's always hard to get out there and get all the nerves out, I started a little slow but I was really happy to get the win."

Seventh seed Donna Vekic also progressed after easing past Caroline Garcia 7-5 6-2. The Croatian will next face Misaki Doi, the runner-up at the Japan Women's Open on Sunday, following her 6-1 6-2 thrashing of Kristina Mladenovic.

Former world number one and two-time grand slam champion Garbine Muguruza is out, however, after squandering a match point on her way to losing to Hsieh Su-wei.

Having won the first set, Muguruza appeared in control but then collapsed in the decider to go down 3-6 7-6 (7-1) 6-1.

The defeat continues a miserable run for the Spaniard, who has not won a match since beating Elina Svitolina in the third round of the French Open, while Hsieh can now look forward to a second-round encounter with Elise Mertens.

"In the first set, she was hitting the ball very hard, and I was not expecting a game like this," Hsieh said. "I was still trying to find a way to get into the match.

"In the second set, I felt a little bit more rhythm, and more stable." 

Nao Hibino, Doi's conqueror at the weekend, followed Muguruza out of the tournament after losing 6-4 7-5 to Varvara Flink of Russia.