SYDNEY -- Casper Ruud has taken up in 2023 where he left off in 2022, beating Brazil's Thiago Monteiro 6-3, 6-2 in the first men's singles match of the New Year to boost Norway at the United Cup mixed teams championship in Brisbane.

Ruud had a successful 2022, rising as high as No. 2 in world rankings before ending the year at No. 3 after reaching the championship match at the ATP finals. His start to last season in Australia was less memorable: he suffered an ankle injury in training and was forced out of the Australian Open a day before his first round match.

On Sunday, Ruud had only a few anxious moments before taking out a must-win match for Norway against Brazil. Norway was 2-0 down when he took the court against Monteiro at Brisbane's Pat Rafter Arena; he left he court victorious 72 minutes later, improving his record against Monteiro to 3-0.

After a comfortable win the in the first set, Ruud went 2-0 down in the second before sweeping six straight games to clinch his win.

"It was a must-win so I was maybe feeling the pressure a little bit," Ruud said. "But I was able to come out with a good start and starting the New Year in tennis down in Australia feels great.

"I always feel very motivated coming down here playing in Australia. Last year was a bit unfortunate. I was not able to play in the Australian Open. I rolled my ankle the day before my first round so I'm very eager to hopefully have a good start of the season down here and this is a perfect way for me to start another year on tour."

Ruud's win only delayed Norway's eventual defeat in Group E. Laura Pigossi beat Ulrikke Eikeri 6-3, 6-4 later Sunday to give Brazil a winning 3-1 lead.

Former Wimbledon champion and Australian Open finalist Petra Kvitova beat Laura Siegemund 6-4, 6-2 to give the Czech Republic an unassailable 3-0 lead over Germany in a Group C match in Sydney.

The match was closer than the scoreline suggests. Siegemund generated 11 break points -- Kvitova saved nine -- and Kvitova overcame 10 double faults to clinch the tie.

Rafael Nadal, who lost his last singles match of 2022 on Saturday to Cameron Norrie will team with Paula Badosa in mixed doubles later Sunday for Spain against Britain.

Two singles matches -- one men's and one women's -- are played over each of two days, with a mixed doubles to conclude the five-match encounter on the second day.

The inaugural United Cup has teams from 18 countries competing in Brisbane, Perth and Sydney through to Jan. 4. The three city champions and the next-best performing country from the group stage will meet at Ken Rosewall Arena from Jan. 6 to 8 to determine the overall winner.