British number one Dan Evans is still waiting for his first win in the French Open main draw after losing to Serbia's Miomir Kecmanovic on day one in Paris.
Evans, 31, had enjoyed his best season on clay going into the Grand Slam, but fell to a 1-6 6-3 6-3 6-4 defeat.
The 25th seed started confidently, but his level dipped as world number 48 Kecmanovic turned the match around.
Evans struggled on serve in the final two sets, winning two of his last 10 service games as the Serb progressed.
Britain's three other representatives in the singles - Johanna Konta, Cameron Norrie and Heather Watson - play their first-round matches on Monday.
Encouraging clay season ends in disappointment on biggest stage
Evans had rarely hidden his disdain for the clay courts previously in his career, but with the help of new coach Sebastian Prieto, who split with fellow Argentine Juan Martin del Potro last year, has grown in confidence on the surface.
More belief in his movement helped the Briton earn a career-high of six wins on the red dirt this year, including a memorable victory over world number one Novak Djokovic in Monte Carlo.
However, Evans could not continue that form on the biggest platform as Kecmanovic won a fluctuating battle lasting more than three hours.
Evans made a blistering start in the Paris sunshine, perhaps showing the benefits of a fortnight away from the match court, playing with quality and variety as he raced into a 5-0 lead.
After having being moved around the court by Evans, and providing few answers, Kecmanovic suddenly found his level as the roles reversed in the second set.
With more than 5,000 fans allowed on site each day as France begins to ease its coronavirus restrictions, Evans began to get distracted by the movement of some spectators - berating a steward for "letting people in" on his second serve - and Kecmanovic took full advantage.
The 21-year-old had only won two of his six clay-court matches coming into the Grand Slam, but grew in confidence as he started hurting Evans with his accurate groundstrokes.
That led to a double break at the start of the third set, before the pair exchanged five more successive breaks - Kecmanovic sealing one in the seventh game at the eighth opportunity - in what became an erratic but entertaining battle.
Evans lost all of his five service games in the third set, but changed the momentum at the start of the fourth with a break and a hold. That grip was quickly loosened, however, as Kecmanovic fought back to win the next five games and close in on his first top-50 win of the season.
A brief rally from Evans geed up the fans on an atmospheric court six and unsettled Kecmanovic as he tried to serve out, only for the Briton to be broken again for the ninth time in the match.
Once the hurt of losing in the Roland Garros first round for the fourth time in his career subsides, Evans will reflect on a positive clay-court season where he has made great strides on the surface.
Then, he will switch attention to the upcoming grass-court season on home soil, where he plays Queen's before aiming to end a run of three straight Grand Slam defeats at Wimbledon.