Roger Federer has said that Lleyton Hewitt was a "big challenge" to him throughout his career.
The Australian reached the second round before losing to Bernard Tomic in his final US Open and will retire after the Australian Open in January.
"He was a big challenge in my career, to understand how in the world can you be so tough physically and mentally at such a young age," Federer told reporters.
"I think he really changed things around and showed me how it's done.
"He made me work harder in practice, get my act together on the court, play tough but fair."
Federer and Hewitt were doubles partners in their Wimbledon debuts in 1999.
