Naomi Osaka said she felt "really scared" after watching opponent Daria Saville collapse with a knee injury just seven minutes into their Pan Pacific Open match in Tokyo on Tuesday.
Australian Saville retired from their first-round encounter after hurting her left knee hitting a cross-court forehand in the second game of the match.
The world number 55 yelled out "my knee" before dropping to the ground in agony, and Osaka rushed over to her side of the court to check on her opponent.
Saville, 28, retired from the match after several minutes of treatment and Osaka, who has struggled with injury herself this year, said she felt "really sad" for her opponent.
"I thought that she was just yelling because she thought her forehand was out, then I realised that she was yelling because she was in pain," said the former world number one, who is aiming to break a slump in form this week in Tokyo.
"Then I got really scared because I felt like as athletes we have a pretty high pain tolerance. It seemed really bad."
Saville eventually got up and walked over to her chair, but she retired after testing her knee with several footwork exercises.
Osaka said she thought Saville was "going to be OK", adding that she was "a fighter".
Osaka, who was on break point at 30-40 in the second game after holding her serve in the first game, will play Brazil's Beatriz Haddad Maia in the second round on Thursday.
"Right now, it still feels a bit weird that I just won a match -- I feel like I didn't win a match," said Osaka, who has lost in the first round at her last three tournaments.
"I guess that I did and I'm progressing through the tournament. I'm not really thinking too much about the final right now."
Osaka is the defending Pan Pacific Open champion, although the tournament is being played for the first time since 2019 because of the pandemic.
Earlier in the day, 2020 Australian Open champion Sofia Kenin of the United States lost 7-6 (9/7), 6-4 to Mexican qualifier Fernanda Contreras Gomez in the first round.
Kenin also reached the French Open final in 2020 but she has since dropped to 315 in the world rankings after struggling with injury.
Number six seed Karolina Pliskova, a two-time Grand Slam finalist, beat Bulgarian qualifier Isabella Shinikova 6-2, 6-1.