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.

"She's one of the best players in the world so you have to be really on top of your game to win," Swiatek said.

"It was always like that, even last season, so I'm not surprised, because I know what she can do. I'm more surprised I wasn't able to step up to her game today."

The defeat is a blow to Swiatek with the Australian Open in Melbourne just two weeks 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.