Jessica Pegula has upset world No.1 Iga Swiatek to give the USA a surprising and significant early lead over Poland in the opening United Cup semi-final in Sydney.

Pegula reversed a run of four straight losses to Swiatek with a commanding 6-2 6-2 victory over the reigning French and US Open champion and hot Australian Open favorite.

Hardly a slouch herself after rising to No.3 in the rankings following a stellar 2022 campaign, Pegula never blinked under a closed roof at Ken Rosewall Arena.

Pegula broke Swiatek five times and dropped her own serve only once in storming to her first win over the three-time grand slam champion since 2019.

'I've played her in a lot of different places but I definitely feel like these were the fastest conditions,' Pegula said.

'Also Poland had a tough turnaround yesterday (flying from Brisbane) and we've been lucky enough to have been playing here, and maybe have a little bit of an advantage in being used to the courts.

'So I wanted to take that and use it as much as I could and I did a really good job playing really aggressive, serving smart and returning really well.'

Undefeated in three singles and two mixed doubles matches at the inaugural United Cup, Pegula's latest win gives the US a 1-0 lead in the best-of-five-match tie.

US Open semi-finalist Frances Tiafoe is a hot favorite to beat the lowly ranked Kacper Zuk in Friday's men's singles to ram home Team USA's advantage ahead of Saturday's remaining three rubbers.

Greece and Italy will play the opening two singles rubbers of the second semi-final in Sydney later on Friday night.

Maria Sakkari faces Martina Trevisan before Michail Pervolarakis takes on Lorenzo Musetti.