Stan Wawrinka bid farewell to the Monte Carlo Masters as the 2014 champion fell 7-5, 7-5 to Sebastian Baez in the first round on Monday.
The 41-year-old three-time Grand Slam winner will retire from tennis at the end of the season and entered the main draw in Monaco as a wild card 20 years on from his debut at the tournament.
The Swiss won his sole 1000-level title 12 years ago in the principality when he defeated his compatriot Roger Federer.
"It was a long time ago, but of course I remember it, it was exceptional," Wawrinka said on court of his 2014 victory after tournament organisers showed a video homage to him.
"To have the chance to play this final here, with this exceptional tournament, and to win it in addition, will always be one of my best memories of my career."
Heading into his meeting with 65th-ranked Argentine Baez, Wawrinka's only match since losing to eventual winner Daniil Medvedev in the last 16 at February's Dubai Open was a three-set defeat to then-world number 348 Matteo Martineau at a challenger event on clay in Naples late last month.
Despite that, Wawrinka raced out of the blocks on Court Rainier III as he took a 4-1 lead in the opening set.
Baez hit back to level scores at 5-5 before pouncing on Wawrinka's serve in the 11th game to get ahead.
Wawrinka then lost his opening service game of the second frame as Baez took control of the match, eventually closing out victory in one hour and 47 minutes after weathering a stirring late comeback by the former world No 3.
As reward for his win, Baez will next face world No 1 and reigning Monte Carlo champion Carlos Alcaraz in the second round.
Elsewhere, three-time former winner Stefanos Tsitsipas lost in straight sets to 16th seed Francisco Cerundolo.
Tsitsipas has not beaten a top-20 player on clay since August 2024.
Andrey Rublev, the 2023 champion, navigated his way through a topsy-turvy opener as the 13th seed beat Nuno Borges 6-4, 1-6, 6-1 to set up a round of 32 meeting with Zizou Bergs.
World number 47 Bergs booked his spot thanks to a 6-4, 6-3 victory against 37-year-old Adrian Mannarino.
Arthur Rinderknech eased past 12th seed Karen Khachanov 7-5, 6-2 and will next play Joao Fonseca after the Brazilian teenager eased past Gabriel Diallo in two sets.
Czech 11th seed Jiri Lehecka needed three sets to see off US qualifier Emilio Nava.