* Stomach virus has disrupted recovery from knee injury
* World number four also pulls out of Qatar Open in Doha (Adds Australian Open reaction)
MADRID, Dec 28 (Reuters) - French Open champion Rafa Nadal has been forced to withdraw from next month's Australian Open because of a stomach virus that has disrupted his recovery from a long-term knee injury, the world number four said on Friday.
"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 practise this past week," the Spaniard, who has also pulled out of the Qatar Open in Doha, said in a statement.
"Therefore I am sorry to announce that I will not play in Doha and the Australian Open, as we had initially scheduled."
Nadal was due to make his competitive comeback after the knee injury sidelined him for six months at this week's Mubadala World Tennis Championship, an exhibition tournament in Abu Dhabi that is not part of the ATP Tour.
The 26-year-old won the event in 2010 and 2011 but withdrew on Dec. 25 citing the stomach virus.
He has not played since June when he suffered a shock defeat in the second round of Wimbledon to Czech Lukas Rosol.
He was subsequently diagnosed with a partial tear of the patella tendon and inflammation in his left knee and was unable to defend his Olympic title at the London Games.
The 11-times grand slam singles champion also missed the U.S. Open and the season-ending World Tour championships before returning to the practice court on November 20.
At last year's Australian Open, Nadal was runner-up to Novak Djokovic after an epic five-set final that lasted almost six hours. It was the longest match at the event and the longest men's grand slam singles final on record.
"It is completely understandable and we really feel disappointed for him," Australian Open tournament director Craig Tiley said on the event's website (www.australianopen.com).
"But without any match practice and without sufficient lead up time on the practice court, it makes it virtually impossible for him to get his body ready," Tiley added.
"We just hope he gets better quickly and we see him back on the tour as soon as possible. Tennis fans across the world have been missing him.
"I am confident we will see him back on the tour soon and back in Australia for 2014."
Nadal, who won a record seventh French Open crown in May on his favoured clay, said doctors had advised a period of rest without any sport for the next seven days starting on Friday.
"As my team and doctors say, the safest thing to do is to do things well and this virus has delayed my plans of playing these weeks," he said.
"I will have to wait until the Acapulco tournament (at the end of February) to compete again although I could consider to play before at any other ATP event.
"I always said that my return to competition will be when I am in the right conditions to play and after all this time away from the courts I'd rather not accelerate the comeback and prefer to do things well."
Nadal's athletic, aggressive playing style places huge demands on his muscles and joints and he has been sidelined several times by injuries during his 11-year career.
He said last week he does not expect to be back to full fitness and close to his best until the Masters event at Indian Wells in March.
"Rafa Nadal suffered last week a viral process that provoked a gastroenteritis with high fever for four or five days," doctor Angel Ruiz-Cotorro, the head of Nadal's medical team, said on Friday.
"Due to this it's been recommended a break from sports for a week.
"Because of this, and considering that the next event is Doha, starting next week he won't be in sufficient physical conditions to continue with his rehabilitation process."
Nadal's uncle and coach Toni added: "We consider it inappropriate to play the Australian Open since we will not have enough preparation for a...grand slam tournament.
"It is simply not conceivable that his first event is a best of five sets event, he wouldn't be ready for that," he added.
"It is true we have been quite unlucky with this but there is nothing we can do. After all this time it is better to do things well and the most professional thing to do is to start when we are ready." (Editing by John Mehaffey)