The American was playing Romania's Sorana Cirstea on Court 17 on Thursday when she appeared to slip as she approached the net at the beginning of the third set.
She fell awkwardly to the grass and, after a moment's silence, screamed out in pain before repeatedly shouting "help me!" as she clutched her right leg.
The 32-year-old was treated by doctors on the court for more than 20 minutes, during which time she was given oxygen and an injection, before she was taken to hospital.
"Bethanie is currently undergoing more scans this morning, and we should know more within the next few hours," a statement from the WTA women's tour said.
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"Bethanie of course thanks everyone for their kind wishes and messages of support."
Mattek-Sands, who suffered a serious ligament injury to the same knee four years ago, has won five grand slam doubles titles with Lucie Safarova and the pair would have held all four at the same time had they clinched a sixth at Wimbledon.
Her Czech doubles partner was reduced to tears at courtside during the incident and led a flood of messages of support for Mattek-Sands on social media.
"I'm deeply hurt after seeing my best friend getting badly injured today ... my thoughts are with her to have a speedy recovery! She is the biggest fighter and one of the strongest people I know! I hope to see that great smile on court again soon!" Safarova wrote on Instagram.
Nine-times Wimbledon champion Martina Navratilova wrote on Twitter: "Am just devastated about Bethanie Mattek-Sands and her injury today - we are all behind you, hoping for the best- lots of love,m."
(Reporting by Alison Williams, editing by Nick Mulvenney)
