PERTH, Australia — Coco Gauff and Taylor Fritz powered the U.S., tournament winner two years ago, into the semifinals of the United Cup mixed teams tennis event Wednesday.

In the first singles match in Perth, Gauff got the ball rolling for her team with a gritty 7-6 (4), 6-2 win over Zhang Shuai. Fritz then made sure of victory by edging a tight contest with Zhang Zhizhen 6-4, 6-4.

“Shuai was playing amazing tennis out there,” world No. 3 Gauff said. “I was literally on the ground for most of the match. The harder I hit, the harder she was hitting.

“I felt like I couldn’t get her on defense. She was playing so aggressive on me, and I felt like I had to go more on my second serves, which brought in more double faults. But if I just put it in, it was a return winner, so I had to take my chances.”

Kazakhstan also advanced to the semifinals with a 2-1 win over a Germany team playing without injured world No. 2 Alexander Zverev.

Zverev was a late scratch for Germany due to a biceps injury. He was scheduled to face Alexander Shevchenko but withdrew at the last minute with Kazakhstan leading the tie 1-0 after Elena Rybakina beat Laura Siegemund 6-3, 6-1.

Shevchenko beat Zverev’s replacement, Daniel Masur, 6-7 (5), 6-2, 6-2 to clinch the tie for Kazakhstan while Zverev watched from the sideline. Germany won the mixed doubles.

Shevchenko took a medical timeout due to heat exhaustion while leading 1-0 in the second set. He came back from the break to dominate the rest of the match.

“It was a heat problem, for sure,” Shevchenko said. “I had a bit of a headache. My head was spinning. It was a struggle this match. To play in this heat was so tough.”

In Sydney, Iga Swiatek and Hubert Hurkacz won their deciding mixed doubles match over the Czech Republic to qualify Poland for the quarterfinals.

World No. 2 Swiatek and Hurkacz beat Tomáš Macháč and Karolina Muchova 7-6 (3), 6-3 in the decider at Sydney’s Ken Rosewall Arena. The Polish team led 5-2 before allowing the Czechs back into the first set.

In earlier singles, Macháč gave the Czech Republic the lead with a 7-5, 3-6, 6-4 win over Hurkacz before Swiatek evened the tie with a 6-3, 6-4 win over Muchová.

Also in Sydney, Katie Boulter’s 6-2, 6-1 win over Olivia Gadecki of Australia helped Britain clinch a quarterfinal place. Boulter went into the match knowing that a straight-sets victory would ensure Britain’s progress to the last eight as Group F winner.

“I’ve been trying to avoid all the stats … but we were at dinner with [former British No.1 Laura Robson] last night, and she thankfully told me I had to win in straight sets,” Boulter said.

Alex de Minaur, who is engaged to marry Boulter, drew the hosts level with a 6-2, 6-1 triumph over Billy Harris before he combined with Gadecki to defeat Charles Broom and Olivia Nicholls 6-3, 7-6 (3) in the mixed doubles. Australia’s 2-1 victory, though, was not enough to qualify the team for the last eight.

In Sydney, Italy earlier won Group D to qualify for the quarterfinals. Winners of the past two Davis Cups and runners-up two years ago to the U.S. in the inaugural edition of the season-opening mixed teams event, Italy won its group without world No.1 Jannik Sinner.

All tournament play shifts to Sydney from Thursday through to Sunday’s final.