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.
Boulter, ranked 51 places below Pegula, came from a set and double break down to win the opening singles rubber 5-7 6-4 6-4.
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 seen GB qualify 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.