Karolina Muchova dismantled world number four Jasmine Paolini in Sydney on Friday before Tomas Machac thrashed Flavio Cobolli as the Czech Republic stunned Italy to reach the semifinals of the mixed teams United Cup.

After taking an unassailable 2-0 lead in the quarterfinals, Czechia will next play top seeds United States on Saturday.

Poland faces Kazakhstan in the other semifinal, which features a mouth-watering women's singles clash between top seed Iga Swiatek and 2022 Wimbledon champion Elena Rybakina.

Tennis powerhouse Italy was favored to progress and had been touted as a title contender even without the presence of men's world No. 1 Jannik Sinner.

But the Italians were left in shock as French Open finalist Muchova gave the Czech Republic the perfect start with a clinical 6-2, 6-2 victory over Paolini in one hour and 43 minutes.

Even though she had been thrashed by Swiatek in the group stage, Muchova entered confident having won all four previous meetings against Paolini.

She unleashed her aggressive game to devastating effect with strong serving and the 28-year-old Czech was ruthless on returns.

"I really wanted to bring a point to the team so I was trying to stay focused," Muchova said. "It was a really tough match. I knew I had to go until the end. She had many break chances. I think I served well. I'm really happy I made it."

It was a disappointing end of the tournament for Paolini after she lost only four games in the group stage. She is hoping to win a maiden first grand slam title after runner-up finishes at last year's French Open and Wimbledon.

Italy's fortunes rested on 32nd ranked Cobolli, but he was completely outplayed by Machac, who produced arguably the most complete performance of his career.

The 25th ranked Czech hit the lines relentlessly and served with considerable power to leave a helpless Cobolli without answers.

Machac had the crowd at Ken Rosewall Arena in the palm of his hands as he sealed the 6-1, 6-2 victory in just 54 minutes. Such was his dominance, a giddy Machac said he was "playing like Novak [Djokovic]".

"I think this quality of tennis is the highest peak I have played," Machac said. "I'm happy to play this match today. I didn't make any mistakes and didn't give him a chance to come back."