Coco Gauff flexed her considerable muscles to give the top-seeded USA a flying start at the $15 million United Cup before teaming up with Taylor Fritz to secure victory over Canada in a nervy mixed doubles battle in Perth.

Gauff picked up from where she left off at the WTA Finals with a clinical 6-3 6-2 win over former US Open finalist Leylah Fernandez at RAC Arena on Sunday night.

The world No.3's impressive victory, completed inside 90 minutes, put the US 1-0 ahead over Canada in their opening Group A tie.

Gauff ended the 2024 season with a bang, backing up her eighth WTA title at the rich and prestigious China Open in Beijing with an even bigger triumph at the WTA Finals in Saudi Arabia.

The 20-year-old took down the second-ranked four-time grand slam winner Iga Swiatek and world No.1 Aryna Sabalenka in consecutive matches en route to the title and shapes as the pair's greatest threat at next month's Australian Open.

Fernandez boasted the second-most hardcourt wins in 2024, behind only two-time AO defending champ Sabalenka, and had upset Paris Olympics silver medallist Donna Vekic in her opening United Cup outing on Saturday.

But Gauff had too much firepower, especially on serve.

The 2022 US Open champion won 92 per cent of points after landing her first serve in the first set, and continued the onslaught in the second to leave Canada in serious danger of losing top spot in their group following a 2-1 victory over Croatia in their opening tie.

"Very happy," Gauff said of her Cup debut.

"Leylah played a great match. I knew going in it was going to be a tough match but I'm super happy with how I played and happy to get a point on the board for my team."

The US were denied outright victory on Sunday, however, when Felix Auger-Aliassime upset world No.4 Fritz in the following men's singles match to pull Canada level.

Fritz, who made his maiden grand slam final in New York in September, boasted a 2-0 win-loss record against Auger-Aliassime but his 24-year-old opponent produced a thrilling comeback when trailing by a set and a break to come through 4-6 7-5 6-3.

But the US, inaugural champions of the tournament in 2023, finally clinched the tie well past midnight as Gauff and Fritz then combined to fend off Fernandez and Auger-Aliassime 7-6 (7-2) 7-5 in the deciding mixed doubles rubber.

The American pair let slip a break lead in both sets, but recovered to seal victory at 12:54 am local time in Perth.

"After the way I lost my singles match it would have been really tough to go home today with a mixed doubles loss today as well," Fritz said.

"Thank you Coco for carrying the team and we got it done."

A delighted Gauff added: "Really happy with how we played today in the mixed.

"I felt we got better the longer the match went. We had a lot of chances and finally we were able to stick 'em when it mattered."