Jelena Ostapenko's French Open title defence came to an end in the opening round as she suffered a shock defeat to Kateryna Kozlova on Sunday.
The world No 5 never looked at ease on the Roland Garros main show court, slumping to a 7-5 6-3 defeat to a Ukrainian opponent who had won both the pair's previous meetings.
Russia's Anastasia Myskina was the only other defending champion to lose in the first round in the Open era in 2005.
By losing, Ostapenko joined an ignominious group including, as well as Myskina, Steffi Graf, Jennifer Capriati, Svetlana Kuznetsova and Angelique Kerber, who all lost as Grand Slam champions on the first run of their defences.
Kozlova, competing with a large, weeping blister on her heel which caused her to take a medical break at the end of the first set, was well worth the victory, playing with power and composure, in contrast to the flustered champion.
Ostapenko's eyes searched the coach's box at every opportunity, but could find no answers as the 20-year-old tumbled out early with 13 double faults amid 48 unforced errors.
Kozlova, the world No 66, will face either former world No 1 Victoria Azarenka or Katerina Siniakova of the Czech Republic for a place in the last 32.