Emma Raducanu came up short in a marathon Washington quarter-final against former world number two Paula Badosa.
The British 21-year-old looked on the brink of reaching her biggest semi-final since her US Open triumph three years ago when she led 3-1 in the third set.
But Badosa battled back in sweltering temperatures to seal a 4-6 7-5 6-4 victory in two and three-quarter hours.
Raducanu, who is still managing her comeback after wrist and ankle surgery, was playing the WTA 500 event after turning down the chance to compete for Team GB at the Paris Olympics.
She did not want to switch back to the Roland Garros clay ahead of her return to the US Open, the scene of her memorable win in 2021.
Having already accounted for Elise Mertens and Peyton Stearns in the American capital, Raducanu began aggressively against Badosa and secured a break of serve in the opening game.
Spaniard Badosa, with whom Raducanu practised and went out for lunch with prior to the tournament, stayed in the rest of the first set but could not fashion a single break point.
Raducanu, looking full of confidence on serve, took her third set point to wrap up the opener in 49 minutes.
When Badosa finally got a look at a break point early in the second, Raducanu snuffed out the danger with an ace down the middle.
But Badosa was applying the scoreboard pressure and at 6-5 she turned up the heat, converting a third set point when Raducanu netted.
After the temperature soared to 36C, the players were afforded a five-minute break before Raducanu emerged to immediately secure a break at the start of the decider.
But Badosa was gifted the break back for 3-3 when a tight Raducanu double-faulted.
Badosa saved three break points in the next, the second with a second-serve ace, and with all the momentum appearing to be on her side of the net she broke for 5-3.
But Raducanu, Britain’s only remaining active player to win a grand slam singles title following Andy Murray’s retirement, had other ideas and saved a match point with a nerveless winner down the line before breaking back.
But a match which deserved a better ending finished when a tiring Raducanu double-faulted on Badosa’s second match point.
Badosa will face world number 49 Caroline Dolehide, who beat fellow American Amanda Anisimova 7-6 (5) 6-1, in Saturday’s semi-final.