World No. 1 Carlos Alcaraz is sweating on his fitness at the Japan Open after injuring his ankle in a 6-4, 6-2 opening win over Argentina's Sebastian Baez on Thursday.

U.S. Open champion Alcaraz crumpled to the ground chasing a shot in the fifth game of the opening set in Tokyo and sat on the court grimacing for several minutes.

The Spaniard had his left ankle heavily strapped during a medical timeout before gingerly returning.