The 30-year-old, whose bustling power game propelled him to 14 grand slam titles, including one at Melbourne Park in 2009, has been blighted by injury since 2012 and in the last two years he has looked a shadow of the player he once was.
He has not advanced to a grand slam quarter-final since Roland Garros in 2015 and last year was dumped out of the Australian Open in the first round by compatriot Fernando Verdasco.
A wrist injury forced him to pull out of last year's French Open before his third round match, ending his pursuit of an record-extending 10th title. He also had to miss Wimbledon.
He returned to win a gold medal in doubles with Marc Lopez at the Rio Olympics but his results after that were disappointing and he ended his season after a second-round loss to Viktor Troicki at the Shanghai Masters in October.
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"I am not injured, no," Nadal told reporters on Sunday when asked about his condition ahead of his first round clash with Germany's Florian Mayer. "Pain-free is a long time ago," he added with a smile.
"Being honest and being realistic, after Roland Garros, the only tournament I played with okay conditions, not 100 percent conditions, was the U.S. Open.
"Because (the) Olympics, even if was a great event, I still had a lot of pain on the wrist. Was so difficult to play."
Having added former French Open and Davis Cup winner Carlos Moya to his coaching team, Nadal said that after the setbacks of the last few years he was just happy to be arriving at the season opening grand slam relatively fit and still eager to win.
"I am here. I am enjoying," he said. "During the last seven months, I played just a couple of matches.
"That's the real thing. I am playing tennis because I am happy doing what I am doing.
"If I don't believe that I can be competitive, and when I mean 'competitive', is fighting for the things that I (fought for) during the last 10 years, I will be probably playing golf or fishing at home.
"I am here because I believe."
(Writing by Greg Stutchbury in Wellington; Editing by Peter Rutherford)
