British number two Jack Draper was eliminated from the Miami Open in the second round after a 7-6 (7-3), 7-6 (7-0) loss to Reilly Opelka.
The 24-year-old lost both tie-breaks to the powerful American, who secured the victory in 93 minutes.
Draper entered the tournament following an impressive run to the quarter-finals at Indian Wells, which included a win over Novak Djokovic, after nearly eight months sidelined with an arm injury.
However, he struggled against the 6ft 11in Opelka, ranked 67th in the world, who delivered 25 aces and 47 winners.
This defeat puts Draper at risk of dropping further in the rankings ahead of the clay-court season, having lost his position as British number one to Cameron Norrie this week.
Meanwhile, Katie Boulter advanced to the third round at Miami Gardens after Denmark's Clara Tauson retired due to injury during their match.
Boulter, the British number three, lost a 5-3 lead as Tauson claimed the first-set tie-break 7-4, but fought back to win the second set 6-4.
After Boulter won the opening game of the third set, Tauson was forced to withdraw.
"It was a complete battle - some ugly tennis and tough conditions as well, but you have to find a way," Boulter said.
"Unfortunately she couldn't finish, but it is what it is. It's never the way you want to win, but I'm happy to be through to the next round."