Cameron Norrie failed to move past the first round of the BNP Paribas Open in Indian Wells after losing to Felix Auger-Aliassime.
The 23-year-old fell 6-3 6-2 to the wildcard in just over hour.
The Briton dropped behind on his first service game in the opening set and failed to convert a break point to get back on serve five games later.
Auger-Aliassime, of Switzerland, broke twice more in the second set and overall won 80% of his first serve point to Norrie's 59%.
Leonardo Mayer overcame wildcard Reilly Opelka to progress to the second round.
The Argentinian battled past Opelka's big serve, prevailing 7-6(6) 7-6(2) despite facing 14 aces from the American.
Both players stayed on serve in the second set to force another tie-breaker, with Mayer outshining the world number 59 to wrap up the victory in one hour and 50 minutes.
Meanwhile, qualifier Marcos Giron, the world number 217, upset Jeremy Chardy, ranked 37th, overcoming the Frenchman 7-6(3) 7-6(1).
The American needed two hours to seal victory after both players held serve comfortably in the first set, with Giron claiming the first break in the fifth game of the second set.
Chardy maintained the pressure, forcing a second tie-breaker but ultimately was outclassed by Giron's serve in the decider.
Elsewhere, Sam Querrey needed three sets to defeat Matteo Berrettini 7-6(5) 2-6 6-4 and Jordan Thompson battled past Federico Delbonis 6-4 6-7(6) 7-6(5).
