Rafa Nadal out of Spain's ATP Cup tie against Australia with back issue

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.