Taylor Fritz edged past Matteo Berrettini 7-6(4) 7-6(6), helping favorites the United States surge to an insurmountable 3-0 lead over Italy in the final of the inaugural United Cup mixed team tournament and seal the title on Sunday.  Coming into the third rubber with his red-hot team up 2-0 and needing one more win to emerge victors in the $15 million tournament, world No. 9 Fritz took the opening set when his big-hitting opponent found the net in a tight tiebreak.  Indian Wells champion Fritz squandered five chances to break in the next set but held his nerve in another tiebreak, ensuring victory for the Americans and sparking wild celebrations on the side of the court and in the stands.  Earlier, Lorenzo Musetti's withdrawal from his match with Frances Tiafoe handed the US a 2-0 lead after Jessica Pegula beat Martina Trevisan 6-4 6-2 at Ken Rosewall Arena.  An injury to his right shoulder forced the 20-year-old Musetti to concede to Tiafoe after his first serve of the second set. The Italian lost the opener 6-2.   "He came out playing really well, he had a chance to go for a break early with some great returns but he may be paid for them," Tiafoe said.   "I thought I was playing at a really high level. I could see he was tugging his shoulder a lot and stayed locked in. I don't want to win this way, especially for my team and myself. I hope Lorenzo feels better."  Tiafoe, ranked 19th in the world, had to fight to win his own opening service game before breaking in the third game and steamrolling his way to a comfortable first set win.   Musetti, however, was struggling as the match wore on and, after attempting to play following treatment at the side of the court, he stepped over the net to concede.   Pegula, meanwhile, notched up a straight sets victory over a battling Trevisan, clinching the opening set 6-4 as both players struggled to hold serve.   She then took command in the second set, winning the first five games before ending a late fightback by Trevisan to secure a straight-sets win.