Heather Watson held serve just twice all match as 16th seed Elise Mertens beat the Briton 6-3 6-4 to reach the third round of the French Open.
The British No 2 lost her first seven service games on Court 3, while an eighth break for Mertens saw the 22-year-old seal the victory after one hour and 27 minutes.
Mertens, who conceded five out of five break points herself, will face either 24th seed Daria Gavrilova or Bernarda Pera in the third round.
An error-prone opening set saw eight breaks, with Watson recording four double faults to Mertens’ three, and 17 unforced errors to the Belgian’s 12.
But crucially it was Mertens who recorded five of those breaks to take the opening set 6-3, with Watson winning just seven points on serve – including one from 15 on her second serve.
Mertens secured a rare hold in the opening game of the second set, only for the pair to trade breaks in the next two games.
Watson was broken for a seventh time as Mertens moved 4-1 ahead, but at the eighth time of asking the Briton finally held serve.
But after battling to level the set at 4-4, Watson could not serve to stay in the match a game later, as Mertens stumbled into the third round at Roland Garros.