Daniil  Medvedev has stepped it up in his US tennis Open semifinal against defending champion Carlos Alcaraz

After the former champ took the first set in a tiebreak... the Russian's dominated the Spaniard in the second set, 6-1.

The winner will face Novak Djokovic in the final, who eased past Ben Shelton in straight sets.

After winning the third-set tiebreak, Djokovic mocked Shelton's celebration... from his opponent's earlier victories in New York... of hanging up an imaginary phone.

It led to a cool handshake at the net at the end of the semi, from the 20-year-old American - who says he'll celebrate how he likes.