She added: “I said, ‘I am here, you are strong, you can do this.’ I was trying, but of course I felt useless.”

Medical attention for Mattek-Sands arrived bit by bit, in what seemed like too slow of a process for Cirstea, who estimated that it took at least 10 to 15 minutes for a stretcher to arrive. Mattek-Sands continued screaming in agony until she was wheeled off the court roughly 20 minutes later.

“Of course you wonder what happen if it was a heart issue or something like this,” Cirstea said.

The tournament defended its response in a statement, saying: “The first response to Court 17 was within 1 minute, by a qualified ambulance technician. The player was kept on court while pain relief was given. The player was then transferred directly into an ambulance and taken under emergency conditions to a hospital.”

Lucie Safarova, Mattek-Sands’s doubles partner, also arrived to the scene, hours before her own singles match was set to begin, and wept by the courtside.

Ranked 103rd in singles, Mattek-Sands is No. 1 in doubles. She won a gold medal in mixed doubles at the Rio Olympics with Jack Sock, and has won the last three Grand Slam events with Safarova.

“It’s just terrible what happened, obviously I’m very sad for her,” Safarova said, speaking barely above a whisper after losing her singles match. “Doesn’t matter about whatever goals we had. It’s just about her being healthy; I know what she has been through with the injuries. Everything was going so well for her.”

The tournament said only that Mattek-Sands had “an acute knee injury.” Safarova, who had been communicating with Justin Sands via text, said she hoped to visit her friend in the hospital as soon as possible.