Gauff laid down an Australian Open marker with a 6-4 6-4 defeat of world No.2 Iga Swiatek before Fritz sealed victory for the top seeds with a 6-4 5-7 7-6 (7-4) win over Hubert Hurkacz in a thrilling men's match.
A mixed doubles rubber wasn't required on Sunday night as Fritz rebounded from dropping the second set to take the match in a deciding tiebreaker at Ken Rosewall Arena.
The stars and stripes lifted the trophy for the loss of just two sets in 14 rubbers, with the rampant Gauff finishing undefeated and looking in ominous touch ahead of the year's first grand slam starting next Sunday.
The world No.3 repeated her straight-sets victory over Swiatek at the WTA Finals in November with another statement win over the five-time grand slam champion.
Both heavyweights had been unbeaten in their first four singles rubbers this week but something had to give when the second and third-ranked women's players clashed.
TOO GOOD FROM GAUFF 🔥— United Cup (@UnitedCupTennis) @CocoGauff | #UnitedCup pic.twitter.com/KzgW6w3ueDJanuary 5, 2025
Swiatek was contesting her first official event since serving a one-month doping ban after the season-ending championship, and the gruelling workload of singles and mixed doubles may have taken a physical toll as Poland fell short in the final for a second year running.
The former world No.1 had her left thigh strapped during Saturday night's semi-final win over Elena Rybakina and needed treatment for a right-side issue late on against Gauff.
Swiatek ended the match in tears after leaving the court briefly following a marathon 10-minute eighth game in the second set, when Gauff pulled out all stops to break back for 4-4.
The 23-year-old duly double-faulted on match point as Gauff rejoiced in posting a second straight triumph over her, having lost 11 of their first 12 meetings.
"I think it's just confidence," Gauff said of her turn-around in fortunes.
"I have the belief that I'm one of the best players in the world and, when I play good tennis, it's hard for me (to lose).
"And today I think I played great tennis and I'm happy to get a point on my board for Team USA."
While Swiatek may enter the Open under a fitness cloud, Gauff will arrive in Melbourne as the hottest player in women's tennis.
The 21-year-old has won 17 of her past 20 matches, starting her run at the China Open before adding a maiden WTA Finals crown to her collection.
Iga Swiatek will head into the Australian Open with her fitness in some doubt. (Mark Evans/AAP PHOTOS)
"I just realised the worst thing that can happen today is that I lose," Gauff said of her positive mindset.
"And especially in this type of format, even if I lose, we still have a chance to win.
"So I'm just going to give myself my best effort and I think that's all the team can ask of me."
Like Gauff, Fritz has started the new year in style after a brilliant end to his 2024 season.
He made his first grand slam final at the US Open, then reached the ATP Finals decider to climb to world No.4 and now also looms as an Open title threat.
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