Australia's Daria Saville has confirmed she tore the ACL in her left knee during her Tokyo Open match against Naomi Osaka.
The Australian's career has been blighted by injury, including long-term knee and Achilles problems, and only this year she returned to the top 100.
Saville yelled in pain after playing a short forehand and collapsed to the court in distress clutching her left knee.
Former world number one Osaka rushed around the net to comfort her fallen opponent, who was able to walk off the court but did not risk continuing, shaking hands after only 11 points.
It ended a four-match losing run for Osaka in unfortunate fashion.
Saville took to Twitter to confirm the extent of her injury.
Osaka has had her own injury problems this year with Achilles and back issues and the former world number one had not won a match since the beginning of August.
"I feel really bad right now because of how it ended," the Japanese player told the fans at Ariake Coliseum.
"I've been injured for most of the year so I just want people to clap for her because she's a really amazing player."
Osaka will take on Brazilian fifth seed Beatriz Haddad Maia in the second round.
Earlier, Australian Ellen Perez was knocked out in the opening round after battling through qualifying to compete in the main draw.
The 26-year-old from Shellharbour won more games than Xinyu Wang, but lost 7-5, 1-6, 6-4.
Perez, rated 352 in the world, twice looked in control against an opponent 274 places above her in the standings, taking the opening three games, then romping the second set.
But her first serve dropped off in the third set, managing to land just 50 per cent, and her Chinese opponent took advantage.
AP/ABC
