Emma Raducanu failed to take three match points as she suffered a third-round exit to world No. 7 Jessica Pegula at the China Open.
The British No. 1 looked set to continue a positive season with a statement win in Beijing.
But, with a place in the last 16 within touching distance, she surrendered a 5-2 advantage and squandered a trio of points for victory in a dramatic second-set tie-break as Pegula dug in to progress 3-6 7-6 (9) 6-0.
Having received a bye through the opening round, Raducanu began the tournament with Saturday's straight-sets win over Cristina Bucsa.
This promised to be a far tougher assignment but the 22-year-old Briton seized the initiative with an immediate break of serve and looked assured for long periods.
Amid some ferocious hitting, Raducanu blasted a backhand winner to bring up set point before completing the job in style with a fine forehand from inside the baseline.
Since beating Emma Navarro in the Miami Open in March, Raducanu had lost eight matches in a row against top-10 players.
Fifth-seed Pegula offered greater resistance in a tighter second set but was forced to battle back from the brink of defeat to force a decider.
World No. 32 Raducanu, watched by her Chinese mother, appeared to have played two of her three match points perfectly but they -- and ultimately the tense tie-break -- somehow went the way of her opponent.
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Momentum quickly shifted in favour of 31-year-old American Pegula as Raducanu's display drastically dropped off.
The 2024 US Open runner-up cruised through the final set and will face 23rd seed Marta Kostyuk of Ukraine in the next round after securing progression in two hours and 21 minutes.
British interest in the tournament now rests on the shoulders of Sonay Kartal.
The 23-year-old, ranked 81st in the world, triumphed 6-3 6-2 against Australian Maya Joint to set up a last-16 meeting with world number five Mirra Andreeva.