Karolina Pliskova's hopes of repeating her 2015 run to the Dubai Tennis Championships final were ended on Monday as the sixth seed bowed out to Coco Vandeweghe in the first round.
Pliskova was runner-up to Simona Halep 12 months ago, but exited at the first hurdle on this occasion as Vandeweghe - who also won when the players met at Wimbledon last year - kicked on after edging a first-set tie-break.
Two breaks of serve in the second set put the American in charge and, after fighting back from 15-40 to move 5-1 up, Vandeweghe enjoyed further success on the Pliskova serve to complete a 7-6 (7-5) 6-1 victory.
Eighth seed Svetlana Kuznetsova fared even worse, the Russian winning just one game in a 6-0 6-1 thrashing at the hands of Julia Goerges.
A three-time finalist in Dubai, Kuznetsova lost the first nine games before finally getting on the board midway through set two.
Madison Brengle was another player to spring something of a surprise.
Brengle, the world number 60, triumphed 6-4 7-5 against Ekaterina Makarova, who is 29 places higher in the rankings, and will now face Petra Kvitova in round two.
Germany's Andrea Petkovic dropped just three games as she eased past Camila Giorgi, while Carolina Garcia was similarly comfortable in beating Anna Karolina Schmiedlova.
Vandeweghe's next opponent will be Kristina Mladenovic, who recorded a 6-2 7-6 (7-1) success against Lesia Tsurenko.