Error-strewn Nadal leads Spain into ATP Cup quarter-finals

World number one Rafael Nadal overcame an error-strewn performance and Yoshihito Nishioka's brave challenge to lead Spain into the ATP Cup quarter-finals on Wednesday.

The 19-time Grand Slam winner made 36 unforced errors before beating Japan's Nishioka 7-6 (7/4), 6-4 in two hours and seven minutes.

Unbeaten Spain -- who won the revamped Davis Cup in Madrid in November -- finished top of Group B with world number 10 Roberto Bautista Agut downing Go Soeda 6-2, 6-4 to secure the tie.

Nadal has yet to drop a set but he looked sluggish at times ahead of the Australian Open, where he aims to equal Roger Federer's record tally of 20 Grand Slam titles.

The long exchanges against an unwavering Nishioka seemed to take a physical toll on Nadal, who appeared weary in Spain's first day-session tie in the hot Perth conditions.

"It's the first time I played under these heavy conditions, so the feeling on court is completely different," Nadal said.

"I played against an opponent who has started the season on fire."