According to The Huffington Post, the withdrawal has nothing to do with knee tendinitis, but rather a stomach virus he picked up prior to a planned warm-up tournament in Doha, Qatar.
My knee is much better and the rehabilitation process has gone well as predicted by the doctors...But this virus didn't allow me to practice this past week, and therefore I am sorry to announce that I will not play in Doha and the Australian Open.
Nadal has been out of commission since Wimbledon and has seen his ranking fall to No. 4 in the world, the lowest it's been since 2010.
The Spaniard was a finalist last year in the tournament and will lose 1,200 points due to his absence. With a good performance from his Spanish compatriot David Ferrer, Nadal could fall to No. 5 in the world for the first time in eight years.
However, Nadal has stated it's not a long-term absence and he plans a return in February.
"I always said that my return to competition will be when I am in the right conditions to play... after all this time away from the courts, I'd rather not accelerate the comeback and prefer to do things well."
The Australian Open will begin on January 14 and the draw will be released on January 8.
The tour and tournament will unfortunately be missing at least one of its brightest stars for a little bit longer.