The 32-year-old’s knee appeared to buckle under her as she slipped during her second-round match with Romanian Sorana Cirstea on Court 17.
In obvious distress, Mattek-Sands screamed out “Help me” as she lay on the turf in front of horrified spectators.
She was attended to by paramedics on court, with an ambulance ready to take her away.
Mattek-Sands had won the first set 6-4 and lost the second on a tie-break moments before the fall occurred.
Safarova went to the court on learning of the injury sustained by her doubles partner and close friend. She was also upset by the scenes as Mattek-Sands received treatment.
Wimbledon said Mattek-Sands had suffered an “acute knee injury”.
Support for Mattek-Sands came in from fellow players.
German Sabine Lisicki wrote on Twitter: “Omg no no no This is so heartbreaking.”
Italian Sara Errani added: “I have no words... be strong @BMATTEK !!! i send all my energy to you.”
Australian Daria Gavrilova wrote: “I know the feeling.... stay strong.”
The Fed Cup account stated: “Our thoughts are with @BMATTEK! We wish you a speedy recovery Bethanie!”