World number one Iga Swiatek was left in tears after a straight-set defeat by American Jessica Pegula in the semi-finals of the inaugural United Cup.

Poland's Swiatek, the French and US Open champion, had beaten Pegula in their previous four meetings.

"It's always hard when you lose, especially when you're playing for the team and your country," Swiatek said.

"So I was just sad. But, you know, it's not the first time I cried after a lost match."

She added: "She played, I think, a perfect match, and it was pretty hard for me to find any spot where I could come back, because she really didn't let me do that.

"I just knew that I felt kind of helpless today, because physically and mentally I wasn't able to kind of show up even, and problem-solve."

The defeat is a blow to Swiatek with the Australian Open in Melbourne 10 days away.

The 21-year-old enjoyed a fine 2022 featuring a 37-match winning streak that enabled her to claim six titles in a row - including the French Open at Roland Garros - before winning the US Open in New York in September.

In Friday's second singles rubber, Frances Tiafoe beat Kacper Zuk 6-3 6-3 to put the United States 2-0 up and on the brink of a place in the final.

Tiafoe, 24, said he had been inspired by Pegula's performance against Swiatek.

"I believe I'm playing some of the best tennis of my life, I'm just having fun out there on court," he said

"Jess is an unbelievable player and after that I was like, 'yeah, I need to come out and put on a show for you guys'."

The tie's final two singles matches - Taylor Fritz v Hubert Hurkacz and Madison Keys v Magda Linette - take place on Saturday. A mixed doubles rubber would then be played if required.

The second semi-final between Greece and Italy begins later on Friday.