The American secured her team’s spot in the final with a 3-6 6-4 6-2 win over Boulter, winning two straight sets after dropping the first.
Boulter was impressive in the opening set and won 13 of her 15 first-serve points, breaking Pegula’s serve twice also.
But she lost her rhythm in the second set and was broken early by Pegula, and despite answering with another break of her own, made four double faults as she lost the second set.
The third decisive set was equally frustrating for Boulter and saw her serve broken early once again, but this time she had no response and Pegula saw out a dominant third set to take the win.
“I think watching my team-mate Emma win earlier definitely motivated me a lot,” Pegula said after the win.
Sonay Kartal had put GB in a difficult position after losing 3-6 6-4 6-3 to Emma Navarro in the opening singles match of the tie.
In what has been an impressive year for Kartal, ranked 82 in the world, she was hoping to add a third win over a top 20 player to her resume.
Both Kartal and Boulter are ranked outside the top 50, compared to Pegula and Navarro sitting at seven and 17 respectively. Despite that, the pair made what could have been routine USA victories tough battles.
Like Boulter, Kartal won her opening set as Navarro started poorly and the Brit played with confidence on the court.
She broke the American in the second set but Navarro responded this time and Kartal’s confidence soon eroded, consistently making errors as she handed the set to her opponent.
The pair fought for control in the decisive set but it was eight consecutive points from Navarro that saw her take the ascendency and finish off her comeback.
“It inspired me a lot the way she turned that match around, go Emma,” Pegula said about her team-mate.
“If it wasn’t for her I wouldn’t have been in such a great position to win the match and beat Great Britain.
“I had to just find a way to compete, I felt like my level wasn’t there. She was playing at a high level, and I was able to turn it around, but I was just competing.”
It was Great Britain’s third semi-final in four years but their agonisingly long wait for a final continues, with their last appearance coming in 1981.
The United States are the competition’s most successful country with 18 Billie Jean King Cup titles, and will face defending champions Italy in Sunday’s final.
How to watch 2025 Laver Cup
The action kicks off at 21:00 UK time on TNT Sports 4 on Friday, September 19.
The second match will follow immediately after, before the action resumes for the second set of matches at 5:00 UK time on Saturday morning.
Coverage of the second day will also start at 21:00 UK time on TNT Sports 3, before coverage of Sunday’s final round of matches gets under way at 20:00 UK time on TNT Sports 4.
