Rafael Nadal pulled out of Spain's opening ATP Cup tie with a lower back problem less than a week before he launches his bid for a record 21st men's Grand Slam title at the Australian Open. The world number two, 34, was scheduled to face Australia's Alex de Minaur at Melbourne Park on Tuesday. "I have a stiff low back," Nadal wrote on Twitter. "Hopefully I'll be better for Thursday." Spain play Greece on Thursday. Spain were runners-up to Serbia in the final