Andy Roddick has admitted that he faces a tough test when he meets Serbian second seed Novak Djokovic in the second round of the Olympics.
The unseeded American defeated Martin Klizan of Slovakia 7-5 6-4 on Monday in what is his first Games since 2004.
"The guy has been the best player in the world over the last couple of years," said Roddick.
"It's going to be tough, but the situation, we've both been through it a million times. I'm going to have to serve well and take some chances on returns."
Djokovic booked his place in the second round following a three-set victory over Italy's Fabio Fognini on Sunday.
