Naomi Osaka began the defence of her Pan Pacific Open crown with a quick win after opponent Daria Saville was forced to retire in just the second game.

Four-time grand slam champion Osaka won the opening game, before Saville was unable to continue due to a scary-looking injury to her left knee.

The former world number one will now face a second round match with world number 16 Beatriz Haddad Maia, who eased to a straight sets victory over Yuki Naito.

Karolina Pliskova secured her place in the next round in Tokyo with a 6-2 6-1 demolition of Isabella Shinikova, while Zhang Shuai will face second seed Caroline Garcia on Wednesday after overcoming Mai Hontama in two sets.

A message from @naomiosaka to @Daria_gav, who was forced to retire from their match through injury pic.twitter.com/EOvdV6512r

— wta (@WTA) September 20, 2022

In Seoul, Jelena Ostapenko's hopes of repeating her 2017 heroics at the Korea Open are still alive after the number one seed narrowly beat 19-year-old Jeong Bo-young.

The world number 19 looked to be cruising after the opening set, but the unheralded South Korean hit back in the second and took it to a tie-break in the third, before Ostapenko's class showed as she won the crucial tie-break 7-2.

Elsewhere, 2021's beaten finalist Kristina Mladenovic saw off wildcard Park So-hyun 6-4 4-6 6-3, while second seed Ekaterina Alexandrova overcame a worrying second set to beat Asia Muhammad 6-1 3-6 6-1.

Wimbledon semi-finalist Tatjana Maria is also safely through to the second round after a tight 7-6 (7-5) 7-6 (7-5) triumph over Eugenie Bouchard.