The 32-year-old player was facing Sorana Cirstea of Romania when her right leg buckled and she fell to the ground.
She called her help right away, but after a few seconds, she began screaming in pain, leading Cirstea to hop over the tennis net and see what she could do to assist.
The American’s cries intensified as she begged: “Help me! Help me! Please!”
Her husband, Justin, and her opponent, were both seen trying to console her.
It took nearly 20 minutes for medical personal to remove Mattek-Sands from the court on a stretcher and place her in an ambulance where she went for help.
“I’m not a doctor but you could see that her kneecap was in a very weird position,” Cirstea told Ben Rothenberg of The New York Times after the incident. “She was shouting, ‘Sorana, help me,’ and all I could say was, ‘I’m here, be strong.’”
Cirstea said she could see Mattek-Sands' knee was "out" like she had never seen a knee before, or perhaps "only in the movies." #Wimbledon— Ben Rothenberg (@BenRothenberg) July 6, 2017
The duo was in the opening game of the third set of a second-round singles match.
Mattek-Sands suffered ligament damage to the same knee years back.