Katie Boulter stunned American world number five Jessica Pegula for the biggest win of her career but was unable to stop Great Britain from falling to defeat at the United Cup.

But the United States fought back, with Taylor Fritz defeating Cameron Norrie.

Fritz and Pegula then beat Boulter and Neal Skupski in the deciding doubles.

Victory over the US would have meant GB qualified for the quarter-finals of the tournament in Perth, but they must now wait to learn their fate with group C opponents the US and hosts Australia set to meet on Monday.

In January 2023, the first edition of the mixed-team competition, Pegula led the US to the United Cup title, but on Sunday she was forced to save three Boulter set points at 4-5 in the opening set.

From there, the 29-year-old won six successive games to take the first set and build a double-break advantage in the second.

But Boulter fought back, winning six of the next seven games to level the match at a set apiece before wrapping up victory in two hours and 44 minutes for her first win over a top-five opponent.

Her previous best wins by ranking were her two victories over then world number seven Karolina Pliskova in 2022, first at Eastbourne and then Wimbledon.

In the second singles match, world number 18 Norrie got off to a slow start as Fritz - ranked eight places higher - raced into a 4-1 lead.

Although the Briton levelled at 5-5, it was Fritz who edged a tight tie-break to win the first set, before the American got the decisive break of serve in the ninth game of the second set to win 7-6 (7-5) 6-4 and send the tie to deciding mixed doubles.

Boulter and Skupski dropped just a single game as they breezed the first set, but the second proved more competitive.

The Americans prevailed in the tie-break, before comfortably winning the match tie-break for a 1-6 7-6 (7-4) 10-7 victory.

Success over Australia on Monday would mean the defending champions progress to the quarter-finals.