Mattek-Sands - one half of the #1 ranked doubles team in the world - was approaching the net when she slipped and immediately started crying and screaming in pain.
Her opponent Sorana Cirstea rushed to her side but she could hardly look as she saw the American's knee "out of place".
Safarova was inconsolable after the horrific injury to her doubles partner.
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She added: "She kept staying, Sorana help me, Sorana help me, Sorana help me. It was like something you see only in the movies", Cristea told BBC News. She was later seen in tears courtside.
She was seen to clutch her right knee for a moment before beginning to scream in pain. Her husband, Justin, was also on the court comforting her. "I said, 'I'm here, I'm here, I'm here.' I think [it] was tough because she was stuck on this, 'Sorana, help me". In this kind of moment when she was screaming so loud, you watch the knee, it was a very uncomfortable moment.
The gangway between courts 16 and 17 was cleared by police and Mattek-Sands, on a stretcher, was wheeled down to an ambulance by Gate Four at 5.10pm.
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The All England Club defended its response time in a series of tweets, saying an ambulance technician was at Court 17 within one minute.
Just a devastating scene at Wimbledon as Bethanie Mattek-Sands goes down with an injury.
Cirstea, who had dropped the first set 4-6 but took the second 7-6 (4), was declared the victor of the second round match.
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