Barbora Krejcikova's French Open title defence ended in the opening round on Monday as local hope Diane Parry roared to a 1-6 6-2 6-3 victory against the second-seeded Czech.
Krejcikova, who defeated Russian Anastasia Pavlyuchenkova in the 2021 final to win her maiden Grand Slam title, arrived at Roland Garros without any competitive matches under her belt since February due to an elbow injury.
But she breezed through the opening set against the 97th-ranked Parry, who was facing a top-10 opponent for the first time in her career, in 34 minutes before her rustiness showed.
The 19-year-old Parry turned the match on its head by winning the second set as errors and double faults began to creep into Krejcikova's game.
Krejcikova took a lengthy break after losing the second set and was booed by the partisan French crowd at the Court Philippe Chatrier on her return.
The 26-year-old Czech got an early break in the deciding set but Parry, urged on by the crowd, levelled things at 2-2 and then got the decisive break in the eighth game.
Krejcikova found the net on match point for her 45th unforced error as Parry completed a remarkable victory with the crowd chanting 'Parry, Parry' throughout the contest.
"It's a dream for me. It was always a dream to play on this court with the French crowd to support me," Parry said during an interview on court.
"They clearly pushed me to victory today. I'm the happiest person right now. It's never easy to start on this kind of court against the defending champion, you can get a bit tight, which happened in the first set but then I managed to relax."
She will play Colombian Camila Osorio for a place in the third round.
Earlier on Monday, Japan's Naomi Osaka crashed out in a 7-5 6-4 first-round loss to American Amanda Anisimova.
The four-time Grand Slam champion never looked comfortable on her least favourite surface against 27th seed Anisimova.
The first three games all went against serve in a tense start but Anisimova held to go 3-1 ahead.
A backhand error by Anisimova allowed Osaka to level for 3-3, but a double fault in the 11th game allowed Anisimova to move 6-5 up and she held serve to love to take the set.
Anisimova broke decisively for 4-3 in the second set as Osaka again double faulted game.
After a couple of match points went begging, Anisimova finished it off with a stunning backhand winner down the line.
Osaka withdrew from the tournament in 2021 after boycotting post-match media duties for mental health reasons.