Rafael Nadal lost his opening match of the season Saturday in the countdown to the defence of his Australian Open title, but women's world number one Iga Swiatek cruised to victory at the United Cup.

Nadal, who was also below-par at the season-ending ATP Finals in November, crashed to Britain's 14th-ranked Cameron Norrie 3-6, 6-3, 6-4 in the mixed-teams tournament in Sydney.

The 22-time Grand Slam champion struggled with his first serve, getting just 58 percent in, which ultimately cost him a first defeat to Norrie in their fifth meeting.

"It's pretty crazy, I never won a set before (against him) so firstly I wanted to do that and I had to stay super patient," Norrie said after the 2hrs 44mins battle at a packed Ken Rosewall Arena.

"It was a super physical match and huge to get through that and finally beat him. He's beaten me easy the last few times."

There was little to separate them in the opening games, with a first break point in game six for Nadal, but Norrie saved with a half-volley.

Nadal's relentless pressure earned him a second and this time the Spanish world number two converted with a searing cross-court forehand before methodically closing out the set in 52 minutes.

But his poor first-serve was an issue and he was broken to go 4-2 behind in the second set. He saved two set points but Norrie nailed a third on his serve to level at one set-all.

Nadal fired consecutive double faults to open set three and while he recovered to hold, his serve once more deserted him in the fifth game when he was broken and the 36-year-old couldn't find a way back.

In Brisbane, Swiatek got Poland off to the perfect start with a dominant 6-1, 6-3 win over Kazakhstan's Yulia Putintseva.

She broke three times in the first set and twice in the second to completely overpower her opponent in just 64 minutes.

"I'm pretty happy with my performance -- first matches of the season are always rusty so I'm happy that in the important moments I was very composed," Swiatek said.

"I feel that representing Poland is always really special. I am willing to focus even more because when I go on court I have goosebumps.

"It's hard to play so well when you're so emotional, but I have to get used to it and I'm really proud to represent my country. I feel like this is a really inspiring tournament."

Crucial moments 

Also in Sydney, former world number two Alexander Zverev slumped to defeat in his first ATP match since tearing ankle ligaments during his French Open semi-final against Nadal in June.

The two-time ATP Finals champion hit eight double faults in losing 6-4, 6-2 to 81st-ranked Jiri Lehecka.

His German teammate Jule Niemeier then lost to Marie Bouzkova 6-2, 7-5 to give the Czech Republic a 2-0 lead in the tie.

Olympic champion Zverev, who has dropped to 12 in the world from a career-high two before his injury, was clearly still struggling.

He fended off break points at 2-3 in the first set but a double fault at 4-5 handed the Czech set point and he grabbed the opportunity, converting with a lob.

With his serve not firing, Zverev was broken to go 3-1 behind in the second set and never recovered.

"I was able to show up in crucial moments. All the experience from last year, losing to good players, gave me the confidence to play against a player like him," said Lehecka, 21.

Brazil, meanwhile, took a 2-0 lead over Norway with Beatriz Haddad Maia and Felipe Meligeni Alves both winning in straight sets.