Melbourne  - Former great Patrick Rafter has voiced his doubts about the desire of deposed Australian Open winner Novak Djokovic after the Serb's retirement during his quarter-final with Andy Roddick.

During a television match commentary, the double US Open champion and one of the most popular of the former Aussie players, had his say about the increasingly controversial Djokovic.

Rafter said that the 21-year-old had a history of "pulling the pin" (with four retirements in Grand Slam matches).

"Everyone wonders when he's going to call the trainer again," said Rafter. "They wonder if he's playing games or is he for real. It's definitely out there and players are aware of him."

Before pulling out against Roddick early in their fourth set, Djokovic had also quit a 2007 Wimbledon final against Rafael Nadal and twice at the French Open (against Nadal and Guillermo Coria).