Jack Draper was dumped out of the Miami Open in the second round after Reilly Opelka took their two sets into tie-breaks and won both.
The mammoth American, 6ft 11in tall, was landing serves that looked almost unreturnable.
Draper had come to Florida buoyed by a run to the quarter-finals in Indian Wells in only his second ATP Tour event following almost eight months out with an arm injury.
But his hopes of a repeat were dashed as Opelka came through the two tie-breaks to win 7-6 (7-3) 7-6 (7-0).
Opelka set the tone with four successive aces in the opening game and he did not let up, finishing with 25 aces and 47 winners.
Draper, who has never made it beyond the second round in Miami, made just six unforced errors but could not find a chink in his opponent's armour.
Speaking to Your Site, the 24-year-old said: "Definitely some disappointment, I never like to lose, but the guy played an incredible match.
"It's a difficult one. Obviously I haven't played a load of tennis, it's still the very start of my comeback from my injury, so it's going to take time."
Defeat leaves Draper at risk of slipping further down the rankings going into the clay-court swing after losing the British No 1 spot to Cameron Norrie this week.
"I've just got to keep on chipping away with my team, trying to do all the right things, practising well," he added. "The thing I'm looking forward to the most is earning the right to get momentum again."
"I didn't have many chances, especially on his serve," Draper told reporters later. "I didn't get broken in the match so that was kind of my job, right?"
Laura Robson, speaking courtside for Your Site:
"One stat Reilly will be aware of today will be the 24 aces which felt like the biggest difference. Coach Jamie Delgado, who was sitting behind me, was telling Jack to give him the wide serve just to give him a different look but even that didn't make a difference.
"It was a tough match for Jack. He was unable to get any rhythm, especially in the return games but that's the challenge of facing somebody who is 6ft 11in and who actually played really well off the baseline.
"I feel like that was the biggest improvement since I last saw Opelka play in person, the fact that he was so loose on the forehand when he needed to be."
Robson added: "Jack was trying so hard to create something when it wasn't really there to be created. He was trying to stay patient. I feel like it's not too much to be upset about if you're Jack and his team. Opelka played too good so onto the next one."
Katie Boulter is through to the third round, though, after opponent Clara Tauson retired early in the deciding set of their rain-hit clash.
The early stages were stop-start because of showers and, when the match did finally get going properly, Boulter was unable to close out a 5-3 lead in the opening set, shouting in frustration after losing in a tie-break.
The British No 3 then trailed 2-0 in the second set but fought back to take it and had won the first game of the third when Dane Tauson, the 17th seed, decided she could not continue because of injury and called it a day with Boulter leading 6-7 (4-7) 6-4 1-0.
"I actually didn't expect it at all," said 29-year-old Boulter. "I think the only time I saw her really struggling was that last game. I wish her the best because it's really not nice to be injured.
"It was a complete battle. There was some ugly tennis but it's ugly conditions as well so you've just got to find a way. Unfortunately she couldn't finish but it is what it is."
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