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World No.1 Swiatek was beaten 6-2 6-2 by the impressive Pegula in just 71 minutes at Ken Rosewall Arena.
Swiatek got the better of Pegula three times last year, but the American made light work of seeing off the three-time grand slam champion this time around to put USA in front.
Pegula struck 21 winners, attacking the Swiatek serve in a commanding display just more than a week before the Australian Open gets under way.
Swiatek's emotions came out after she was emphatically defeated by the world No.3 in Sydney.
Pegula said: "I played her today, conditions are totally different than anywhere else I played her. Obviously maybe I had a little bit of an advantage, they just flew in yesterday.
"Definitely the fastest conditions I have played her [in]. Every other place I've played her has been pretty slow. I think that favoured me a lot, and I was able to use that to my advantage and play a really super clean match.
"I think I just was able to execute my game plan probably better than I have previous times."
Frances Tiafoe backed up Pegula's win by beating Kacper Zuk, who replaced the unwell Daniel Michalski, 6-3 6-3.
That ensures Taylor Fritz has a chance to put United States into the final if he beats Hubert Hurkacz on Saturday.
Italy is in control of the other semi-final after victories for Martina Trevisan and Lorenzo Musetti.
Trevisan upset Maria Sakkari 6-3 6-7 (4-7) 7-5 before Musetti outclassed Stefanos Sakellaridis, winning 6-1 6-1.