Taylor Fritz and Coco Gauff teamed up to guide the United States through an exhilarating United Cup tie against Canada in Group A on Sunday.

In a winner-takes-all mixed doubles clash, Gauff and Fritz earned a hard-fought 7-6 (7-2) 7-5 victory over Felix Auger-Aliassime and Leylah Fernandez, sealing a 2-1 triumph.

Fritz had lost his singles clash with Auger-Aliassime, but made amends for his earlier display, with the Americans recovering after letting two break leads slip in both sets.

"Really happy with how we played today in the mixed," Gauff said. "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."

"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 added.

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

In the opening match, Gauff picked up where she left off from last season by comfortably dispatching Fernandez to seal a 6-3 6-2 win, losing just four points on her own serve.

But Auger-Aliassime levelled the tie and kept Canada alive with a comeback victory over Fritz, his first triumph over the American after three previous meetings.

The Canadian prevailed after two hours and 18 minutes, emerging a 4-6 7-5 6-3 victor, but he was unable to beat Fritz again in the doubles soon after.

In Group E, Alexander Zverev produced a commanding straight-sets triumph over Thiago Monteiro to steer Germany to victory over Brazil after Laura Siegemund's earlier win.

Zverev claimed immediate breaks in both sets to ease to a 6-4 6-4 victory, serving 11 aces and dropping just three points on his first serve in the one-hour and 17-minute clash.

"I'm very happy to be back playing for Team Germany again," Zverev said. "Perth is one of my favourite cities in Australia, and I'm enjoying my time on the court.

"I think this is the first time we've been up 2-0 in a tie. Last year we played a deciding match every single time, so it's nice to know that we've got this one in the bag already."

Siegemund's win was less straightforward, as she outlasted Beatriz Haddad Maia to emerge a 6-3 1-6 6-4 victor in a gruelling affair.

She played for over two-and-a-half hours before crossing the finishing line, claiming her 20th top-20 win, coming from behind in the first set by winning nine consecutive games.

Germany completed a 3-0 sweep of the tie when Siegemund teamed with Tim Puetz to defeat Rafael Matos and Carolina Alves 7-6 (10-8) 6-4 in the mixed doubles rubber.

Italy, meanwhile, also made a winning start, with Jasmine Paolini earning her first victory of the new season by comfortably beating Belinda Bencic.

Paolini claimed a 6-1 6-1 victory in her first match since leading Italy to the Billie Jean King Cup title in November, with this win her first over the Swiss at the third attempt.

Flavio Cobolli had put Italy into a 1-0 lead earlier in the day after edging past Dominic Stephan Stricker 6-3 7-6 (7-2) following a breakthrough year on the court.

Following Paolini's win, Sara Errani and Andrea Vavassori completed the sweep with a 6-4, 6-4 victory against Bencic and Stricker.

In Norway's first tie of their campaign, Casper Ruud's enthralling 7-6 (8-6) 5-7 6-4 win over Tomas Machac mattered little as his country were beaten by the Czech Republic.

Karolina Muchova got her nation off to a blistering start by brushing aside Malene Helgo 6-2 6-2 in one hour and 17 minutes before Ruud levelled up the tie soon after.

But Machac recovered from his earlier defeat to send his country top of Group B, teaming up with Muchova to beat Ulrikke Eikeri and Viktor Durasovic 6-4 6-4.