Sloane Stephens went from beast mode to feast mode after her win at the US Open this weekend ... chowing down on some fancy steak to celebrate.

Sloane hit up STK Downtown Saturday in NYC to celebrate her win against Madison Keys, and ate like the champ she is! Our steak sources tell us she brought about 70 people for dinner around 11 PM. The group ordered up tuna tartare, STK's signature sliders, shrimp cocktails and, of course, several steaks.

Eyewitnesses tell us Sloane passed on the booze, but fully indulged in chocolate chip cookies.

To the victor goes the spoils ... and the calories!