The world No. 3 was dumped out of the season opener by 34-year-old German qualifier Michael Berrer 1-6, 6-3, 6-4 in a match lasting a little under two hours.
Nadal, who defeated Gael Monfils in the final 12 months ago, admitted to being nervous in his first tour match since undergoing appendix surgery last November.
"I was playing with more nerves than usual. I knew that winning a couple of matches here will help me, so that makes me play a little bit more under more tension," Nadal said, the ATP Tour website reported
His second defeat inside a week -- Nadal lost in straight sets to Andy Murray
at an exhibition tournament in Abu Dhabi on January 3 -- has cast doubt in his own mind if he is ready for the season's opening grand slam in Melbourne which gets underway on January 19.
"This is the third comeback of my career after injury, so we will see how it goes in a couple of months. I wanted to start well, but it was not the case today," Nadal said, Agence France-Presse reported.
"I also want to play in the Australian Open which is a tournament I like a lot. I don't know if that will be the case either, but I am going to work for it. If not I will be playing at Rio and Buenos Aires (ATP Tour clay court tournaments taking place in February).
"The season is long. After coming back from injury you can't see the immediate moment, you have to look further ahead a little bit."
After suffering a back injury in last year's Australian Open final defeat to Swiss Stanislas Wawrinka, and missing the U.S. Open in September with a wrist injury, Nadal could be forgiven for being cautious about his return.
Tsonga out of Melbourne
While Nadal's participation remains in doubt, Jo-Wilfried Tsonga will definitely not be in the draw for Melbourne.
The 29-year-old Frenchman, who reached the final in 2008, has yet to recover from an arm injury sustained during France's defeat to Switzerland in the Davis Cup final last November.
"It is with great disappointment that I have to postpone today the start of my season, pulling out of the Australian Open," Tsonga said Wednesday.
"I'm still suffering from a forearm inflammation (intersection syndrome) and that prevents me from being at full capacity."
There seem to be no such health worries for world No. 1 Novak Djokovic who was forced to withdraw from last weekend's final against Murray
in Abu Dhabi with a fever.
The Serb cruised through his opening match in Doha, seeing off his compatriot Dusan Lajovic 6-2 6-1 in 59 minutes.
Djokovic will play Ukraine's Sergiy Stakhovsky in round two.