World number two Rafa Nadal has pulled out of Spain's opening ATP Cup tie in Melbourne against Australia with a lower back problem, the 20-times Grand Slam winner said on Tuesday.

"Hi all, we have decided with #TeamSpain and my team, to not play today the first match of the @ATPCup here in #Melbourne since I have a stiff low back," Nadal said on Twitter.

"Hopefully I'll be better for Thursday." 

The tournament is one of a slew of warmups being played at Melbourne Park in the run-up to next week's Australian Open.

The first tennis Grand Slam of the year will take place from February 8 to 21 at Melbourne Park.