Team USA opened its United Cup title defense with a 2-1 victory over Great Britain in Group C on Sunday, but not before Katie Boulter's earlier heroics left the Americans sweating.

Boulter pulled off the win of the United Cup so far with a 5-7, 6-4, 6-4 victory over No.5 Jessica Pegula to start off the tie, coming from a set and double-break down to earn her first Top 5 win.

After leading the U.S. team to the title 12 months ago in the competition's first edition -- she went 8-1 across singles and doubles last year -- Pegula looked on course for a fairly uncomplicated win to open the Americans' title defense. In the match's first hour, she turned a 4-2 first-set deficit to a 7-5, 3-0 lead -- winning six straight games after saving three set points on serve at 4-5.

But Boulter caught fire from a set and double-break down, and after 2 hours and 44 minutes, scored best career-win by ranking. Her best previous win by ranking came in 2022, when she beat then-World No.7 Karolina Pliskova twice in a month's span on grass in Eastbourne and at Wimbledon.

"I felt like I really wanted to take it to her today and give it my best shot and see where my level was at," Boulter said. "I think I surprised myself a little bit at times with how well we were both competing, and we had some really good points. I felt like I just backed myself at the right times and went after it."

After the first two hours of play and two split sets, Pegula broke serve to start the decider but never again led. Boulter responded immediately, and neither woman had break point until the Brit seized a 30-40 chance in the eighth game. Though she failed to serve out victory, she wouldn't be denied, and broke for an eighth and final time to seal the upset.

Twenty forehand winners -- and 23/30 points won at net -- helped Boulter's cause.

Taylor Fritz kept the United States alive when he defeated Cameron Norrie 7-6(5), 6-4, gaining revenge for his defeat to the British lefty at the mixed-teams event last season.

Meeting for the 14th time in their Lexus ATP Head2Head series, Fritz and Norrie exchanged heavy blows in the hot Perth conditions. Fritz recovered from squandering a 4-1 lead in the first set, sealing the opener in the tiebreak. He then gained the decisive break of the second set in the ninth game, advancing after one hour and 47 minutes. The World No. 10, who received medical treatment on his left thigh during the match, now leads Norrie 8-6.

The mixed doubles decider pit Pegula and Fritz against Boulter and Neal Skupski. After a slow start, the Americans came from a break down in the second set to force a deciding match tiebreak. With a spot in the quarterfinals on the line, Great Britain could not close out the win. Pegula and Fritz held their nerve from 4-4 to seal the edgy 1-6, 7-6(4), [10-7] win.

The victory dropped Great Britain to 1-1. The United States (1-0) will now qualify for the quarterfinals if they beat Australia (0-1) on Monday night. The winner of Group C will advance to the quarterfinals in Perth. 

The United States is aiming to defend its title at the 18-country mixed-teams event, having won the crown in 2023.