American tennis star Jack Sock may have lost his Hopman Cup tie with Lleyton Hewitt on Tuesday night – but he walked off court a winner with the Perth crowd.

That was after he showed an incredible display of sportsmanship that had even Australia's retiring veteran in shock.

Serving to stay in the first set, trailing 4-5, Hewitt's first serve at 30-0 was called out by the chair umpire.