Johanna Konta was unable to take either of two match points as she lost a marathon encounter to Daria Kasatkina in the second round of the Dubai Duty Free Championships.
The British number one led by a set and 3-1 and looked on the brink of victory at 6-4 in the second-set tie-break, but Kasatkina won four straight points, taking the set with a winner that just clipped the baseline.
The 20-year-old then proved the more consistent in the decider, wrapping up a 4-6 7-6 (8/6) 6-2 victory after two hours and 59 minutes to set up a quarter-final against countrywoman Elena Vesnina.
Although Konta led by a set and a break, the match was extremely close, with Kasatkina, who is ranked 24, failing to take any of seven break points in the opening set. When she did finally convert to pull level at 3-3 in the second set, it was on her 10th opportunity.
Top seed Elina Svitolina survived a tricky tussle with lucky loser Wang Qiang to keep alive her hopes of back-to-back titles at the Dubai Championships.
The Ukrainian, who defeated Caroline Wozniacki in the final last year, lost the second set to China's Wang but recovered to win 6-1 5-7 6-2 and set up a quarter-final clash with Japan's Naomi Osaka, who defeated Anett Kontaveit 6-2 7-6 (7/5).
Second seed Garbine Muguruza did not take to the court until after midnight for her match against American teenager Cici Bellis and finished off a 6-3 7-5 victory just before 2am.
Fourth seed Jelena Ostapenko also fell, losing to Elena Vesnina, but there were wins for Karolina Pliskova and Angelique Kerber, who will play each other in the last eight, and fifth seed Caroline Garcia.
Defending champion Timea Babos was in a ruthless mood at the Hungarian Ladies Open.
The third seed and local favourite defeated Tatjana Maria 6-1 6-0 in less than an hour in her opening match to set up a second-round clash with Mona Barthel.
Top seed Dominika Cibulkova and second seed Zhang Shuai were also comfortable winners but Alison van Uytvanck defeated fifth seed Mihaela Buzarnescu 6-1 6-0 and Petra Martic was a 3-6 6-4 6-4 victor against sixth seed Aliaksandra Sasnovich.