MELBOURNE, Australia -- A bout with the flu caused top-seeded Novak Djokovic to cancel a training session on the eve of the Australian Open, joining a growing list of top-seeded players on the men's side to be affected by illness and injuries.

A spokesman for the International Tennis Federation, which handles media relations for the ATP Tour at Grand Slam tournaments, said Sunday that Djokovic "was not well" on Saturday and that he canceled the practice session at Melbourne Park and a pre-event news conference.

Novak Djokovic said Sunday he might have picked up a virus when he played recent matches. Jerry Lai/USA TODAY Sports

The Serbian star scheduled another training session for Sunday afternoon, after which he appeared fit and was hitting the ball hard and moving well around the court. He was watched by his coach, former German star Boris Becker.

Djokovic said later Sunday he might have picked up a virus when he played recent matches in Abu Dhabi and Doha, Qatar.

"I had a tough couple of days, it's all behind me now," Djokovic told a media conference Sunday evening.

"It was a little bit of a cold, and flu, stomach ... maybe carried that from the Middle East, but now it's passed."

Djokovic, a four-time winner of the Australian Open, is not scheduled to play his first-round match until Tuesday against Slovenian qualifier Aljaz Bedene of Slovenia.

On Saturday, No. 3-ranked Rafael Nadal told media at a news conference that "I would be lying if I say I feel that I am ready to win." Asked what aspect of his game he was happiest with, Nadal replied: "Nothing."

Also Sunday, 2009 US Open winner Juan Martin del Potro of Argentina withdrew to avoid reinjuring a surgically repaired wrist that he said had been hurting him for the "last couple days and weeks."

Del Potro made it the quarterfinals of the Sydney International last week, his first tournament after 11 months off the tour.

The Associated Press contributed to this report.