Jessica Pegula had to regroup after being upset in her singles match to help the United States defeat Great Britain in their United Cup tie Sunday in Sydney.

Katie Boulter stunned the fifth-ranked Pegula by rallying from a set down to win 5-7, 6-4, 6-4 in nearly three hours.

The Americans drew level when 10th-ranked Taylor Fritz defeated Cameron Norrie 7-6 (5), 6-4 to force a deciding mixed doubles match. Pegula and Fritz then combined for victory over Boulter and Neal Skupski 1-6, 7-6 (4), 10-7 that sealed the tie for the defending champions.

The Americans will qualify for the quarterfinals if they win their final match against Australia. All three teams still have a chance to qualify if Australia beats the U.S. in the final group match.

In other results, 2021 US Open finalist Leylah Fernandez led Canada to a 2-1 victory over Chile in its opening United Cup match.

Fernandez defeated Daniela Seguel 6-3, 6-2 before Nicolas Jarry leveled the Group B tie for Chile by beating Steven Diez in straight sets. Diez had replaced Felix Auger-Aliassime, who was rested with a minor knee issue.

Fernandez and Diez then won the deciding mixed doubles against Seguel and Tomás Barrios Vera 7-5, 4-6, 10-8 to seal the tie for the Canadians.

“I’m extremely happy with the way I played,” world No. 35 Fernandez said after her 73-minute win over Seguel. “It wasn’t the greatest match because it is my first match of the season, the nerves, but I’m extremely happy with the way that I was able to manage it.”

Later Sunday, men’s No. 1 Novak Djokovic returns to Perth for the first time since the 2013 Hopman Cup when he leads Serbia in its first Group E match against China.

China won its opening tie against the Czech Republic 3-0 and would clinch a quarterfinals berth with a victory over Serbia.

Mixed teams from 18 countries are playing in Perth and Sydney in what doubles as a warmup for the Australian Open.

The Associated Press contributed to this report.