World number one Rafael Nadal is confident and ready for the Australian Open amid fitness concerns, according to his coach Carlos Moya.

Despite worries about the Spaniard's knee, Nadal practiced on Rod Laver Arena Friday.

The 16-time grand slam champion cast doubts over his fitness after withdrawing from the Brisbane International.

However, the 31-year-old will be ready for the year's first major beginning January 15, unless something changes.

"The two trainings have been very positive after arriving late [Thursday]," Moya told El Espanol.

"We want to make a small adaptation to the weather and also to the court. Starting [Saturday], he will start playing training sets and he will continue to do so in the coming days.

"Rafa is fine, with confidence and playing at a good level. We are optimists.

"We think he will be fine to play the tournament, if nothing gets twisted or changed."

Champion in Melbourne in 2009, Nadal has also been runner-up three times, including losing a thrilling final to Roger Federer last year.

His fitness is a boost to the Australian Open, with Andy Murray and Kei Nishikori having already withdrawn, along with Serena Williams.