World number two Rafael Nadal pulled out of Spain's opening ATP Cup tie against Australia Tuesday, complaining of a stiff lower back.

The 20-time Grand Slam champion was due to take the court against Australia's top-ranked player Alex De Minaur, but opted to rest instead as he prepares for the Australian Open next week.

"Hi all, we have decided with Team Spain and my team, to not play today the first match of the ATP Cup here in Melbourne since I have a stiff low back," he tweeted. 

Nadal added that he hoped to have recovered by Thursday, when Spain meet Greece.

Spanish number two Roberto Bautista Agut will now face de Minaur, while John Millman takes on Pablo Carreno Busta in the opening rubber.

Nadal, who is aiming for a record-breaking 21st Slam title in Melbourne, moving him past the injured Roger Federer, played an exhibition in Adelaide on Friday, beating Dominic Thiem 7-5, 6-4.