ATP Cup: Rafael Nadal pulls out of opener with back problem

Rafael Nadal is aiming to overtake Roger Federer by winning a record 21st men's Grand Slam title this monthRafael 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.

"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 of last year's inaugural ATP Cup.

The men's team event, which has been reduced to 12 teams from 24, is being played at the same time as five other tournaments at Melbourne Park this week after a strict quarantine for all players who…