The USA have surrendered their chances of winning the Hopman Cup with the withdrawal of John Isner because of a knee complaint.

Isner took to the court on Tuesday with strapping on the knee, saying after the match he was suffering from tendonitis.

He will be replaced by South Australian teenager Thanasi Kokkinakis in their tie against Spain today.

Isner's withdrawal means Spain automatically qualify for Saturday's final.

Kokkinakis is ranked No.756 in the world, the second highest ranked 16-year-old.