Germany's United Cup title defence is over after they slumped to a quarter-final defeat to Kazakhstan on the back of Alexander Zverev's injury withdrawal.

World No.2 Zverev pulled out of Wednesday's tie at Perth's RAC Arena as a precaution, citing a minor biceps niggle.

Kazakhstan shot out to a 1-0 lead after world No.6 Elena Rybakina posted a 6-3 6-1 win over Laura Siegemund in the women's singles.

Zverev's replacement, world No.254 Daniel Masur, threatened to pull off an upset in the men's singles as Alexander Shevchenko began to wilt in the 36C heat.

But Shevchenko overcame his physical struggles to post a gutsy 6-7 (5-7) 6-2 6-2 win and secure a place in the semi-finals for Kazakhstan.

Germany won the dead rubber mixed doubles contest 6-2 6-2.

Shevchenko had a medical assessment due to heat exhaustion while leading 1-0 in the second set of his singles match, and he questioned why the roof stayed open in such oppressive heat.

"After the match, I was feeling like I was going to die," Shevchenko said.

"They were brutal conditions. I don't understand why we are not closing the roof in these conditions, because it's quite dangerous if you're going long matches with this heat.

"I had a headache, shortness of breath - everything.

"The more concerning was the head was spinning - dizzy. So my concentration was off.

"But it got better because I switched up my game as well. I started to play more aggressively."

Zverev looked in ominous touch on the way to posting a straight-sets win over Brazilian Thiago Monteiro in Germany's tournament opener on Sunday.

But late in the third set against China's Zhizhen Zhang on Monday, Zverev could be seen stretching out his right arm.

He fought on to post the 2-6 6-0 6-2 victory, but he was unable to back up for Wednesday's clash.

Despite not featuring in Germany's quarter-final loss, Zverev isn't in any doubt for the Australian Open.

Rybakina put Kazakhstan in the box seat against Germany with a 64-minute rout of world No.80 Siegemund.

The 2022 Wimbledon champion converted four of her seven break-point opportunities across the match, and fired six aces to nil.

Siegemund committed 22 unforced errors and wasn't able to convert either of her two break-point chances in a match that began in 30C heat.

In Sydney, world No.2 Iga Swiatek and Hubert Hurkacz launched Poland to the quarter-finals after a 7-6 (7-3) 6-3 win in the mixed doubles.

Swiatek had levelled the tie with a 6-3 6-4 defeat of Czechia's Karolina Muchova after Polish men's No.1 Hurkacz lost to Tomas Machac 7-5 3-6 6-4.

Aiming to do better than last year's second-place finish to reigning champions Germany, Poland will face Great Britain in the quarter-finals on Thursday night.

Team GB topped Group F despite losing 2-1 to Australia in Wednesday night's Sydney session.

The tournament hosts needed to beat Britain 3-0 to advance but twin wins in singles and mixed doubles for Alex de Minaur were not enough after the Australian spearhead's fiancee Katie Boulter crushed Olivia Gadecki 6-2 6-1 in the women's singles.

De Minaur will nevertheless head to the Australian Open in a buoyant mood after an undefeated Cup campaign, including a 6-2 6-1 victory over Billy Dowling on Wednesday night.

He rounded out his Open preparation with a rousing 6-3 7-6 (7-3) mixed doubles triumph with Gadecki.

In the late tie in Perth, the star-studded top-seeded Team USA lived up to their title favouritism to join Kazakhstan in the semi-finals with a 2-0 win over China.

World No.3 Coco Gauff eased past Zhang Shuai 7-6 (7-4) 6-2 before men's world No.4 Taylor Fritz sealed the deal for the Americans with a 6-4 6-4 defeat of Zhang Zhizen.