Jessica Pegula gave the United States the perfect start to their United Cup semi-final against Poland with a 6-2 6-2 victory over world number one Iga Swiatek in the opening singles rubber on Friday before Frances Tiafoe doubled their advantage.   Swiatek was only able to hold her serve once in the first set as Pegula raced to a 5-0 lead and the American world number three kept up the pressure with some brilliant returns in the second set.  Pegula secured a dominant victory that will also give her a personal boost heading into this month's Australian Open when Swiatek sent a forehand wide after little more than an hour on Ken Rosewall Arena.  "I thought I did a good job playing really aggressive, serving smart and returning really well," said Pegula, who had lost her previous four matches against Swiatek.   It was her first career victory over a reigning number one and a result that left Swiatek in a flood of tears in Poland's dugout after the match.   "She's one of the best players in the world so you have to be really on top of your game to win against Jessie," 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."   Tiafoe gave the Americans a 2-0 lead when he comfortably beat Kacper Zuk 6-3 6-3, saying after the match that Pegula's performance had inspired him.   "It was great, I was so amped watching. Forget about me playing, I was so happy for Jess ... I know what it means for her going forward," said Tiafoe, who has won all four of his singles matches in the event, dropping one set.   Maria Sakkari will be in action in the evening session when Greece take on Italy in the other semi-final at the inaugural mixed team tournament.