Tennis Star Juan Martin del Potro Hits Miami With Thong'd Out Model GF

Here's tennis star Juan Martin del Potro showing us how he's recovering from that pesky knee injury ... a Miami beach romp with his model girlfriend!

Feelin' better already?!

Juan -- the former #3 ranked player in the world -- fractured his kneecap earlier this year after slipping on grass during a tourney and had to undergo surgery to repair it.

He's hoping for a big-time comeback when he's all healed up -- but in the meantime, the 31-year-old Argentinian stud is relaxing in Miami with actress/model Sofia Jujuy Jimenez.

They reportedly started dating earlier this year -- and it seems to be going REALLY well ... at least judging by the PDA in the pics!

From what we can tell on her IG page, Sofia is a pretty good model ... meaning, she looks damn incredible in a bikini.