Coco Gauff and Taylor Fritz powered the United States into the United Cup semi-final as they eased past China with comprehensive singles victories. 

Gauff set the tone for the USA on New Year's Day in Perth, opening the United Cup quarter-final with a gritty comeback against Zhang Shuai.

The 20-year-old erased three early breaks before cruising to a 7-6 (7-4) 6-2 triumph, improving her record against her Chinese opponent to 3-0 to put the USA in control.

Fritz then produced a clinical serving display to defeat Zhang Zhizhen 6-4 6-4 to move his side into an unassailable lead, dropping just four points behind his serve. 

"Coco did such a good job putting us ahead and giving me the chance to come out and put us through," said Fritz. 

"There was a little bit of pressure because I didn't want to be the one who loses it for the team after Coco won all her matches this week."

In her third appearance in the USA United Cup team, Desirae Krawczyk teamed up with debutant Robert Galloway to play her first match in the event.

The American duo overcame Zhang and Sun Fajing 6-3, 6-7 (1-7) 10-3 to make it a 3-0 sweep in the quarter-final tie before they head to Sydney.

In the other last-eight tie, Elena Rybakina helped lead Kazakhstan to the semi-final of the United Cup while also ending Germany's title defence in the process.

The former Wimbledon winner registered another comprehensive victory to give Kazakhstan the advantage, beating Laura Siegemund 6-3 6-1 in the opening match.

Germany then suffered a further blow, with world number two Alexander Zverev withdrawing from the tournament due to a bicep strain moments before his match.

His replacement, Daniel Masur, got off to a flying start and edged the first set, but Alexander Shevchenko rallied from behind to clinch a 6-7 (5-7) 6-2 6-2 victory to give Kazakhstan an unassailable lead. 

Germany did, however, restore some pride in the mixed doubles, with Siegemund and Tim Puetz beating Zhibek Kulambayeva and Dmitry Popko 6-2 6-2. 

Iga Swiatek and Hubert Hurkacz clinched Poland's place at the top of Group B, defeating the Czech Republic's Karolina Muchova and Tomas Machac to win the tie 2-1. 

Swiatek and Hurkacz emerged 7-6 (6-3) 6-3 victors in the deciding doubles encounter, with Poland returning to the quarter-final at the competition. 

Machac had moved the Czech Republic to the brink of the last eight at the start of the tie with a tense win against Hurkacz, triumphing 7-5 3-6 6-4 in just under two hours. 

Swiatek, however, levelled up the tie with an enthralling 6-3 6-4 victory over Muchova, with the Pole made to work for her victory over the world number 22. 

Swiatek became the first female or male player to secure 10 singles match wins at the United Cup, with last year's runners-up sealing their progress in the doubles. 

"Honestly, I was sure they were going to play amazing tennis because they have always great teams in doubles and mixed doubles," Swiatek said.

"They are kind of raised that way. But I knew that we have the game to win it. For sure, we were leading from the beginning.

"In the first set it got tight at the end, but I felt that we were leading anyway, so in the tie-break we just used that. In the end, it seemed that we had everything under control."

Poland's opponents in the quarter-final will be Great Britain, despite losing to Australia in their Group F clash on Wednesday. 

Argentina, Great Britain and Australia all finished with a record of 1-1, but Katie Boulter's win over Olivia Gadecki proved decisive in their progression. 

Boulter knew a straight-set victory from any of the three rubbers against the hosts would take Great Britain through as Group F winners, and she did just that. 

In the first match of the tie, Boulter won 6-2 6-1 inside 70 minutes to secure qualification before Australia won the next two encounters.

Alex de Minaur, Boulter's fiancé, thrashed Billy Harris 6-2 6-1 before joining up with Gadecki in the mixed doubles and beating Charles Broom and Olivia Nicholls 6-3 7-6 (7-3).

"I've been trying to avoid all the stats on all this, but we were at dinner with Laura Robson last night, and she thankfully told me I had to win in straight sets," Boulter said.

"So I was feeling the pressure from last night, so I really appreciate that, Laura."