Britain's Harriet Dart raced into the US Open second round but qualifier Jan Choinski ran out of steam in a five-set thriller on his New York main-draw debut.
Dart, the British women's number three, claimed an assured 6-1 6-2 victory over Chloe Paquet of France.
But Choinski, who won three qualifying matches to reach the main draw, lost 6-2 6-3 5-7 6-7 (5-7) 6-3 to Spain's Roberto Carballes Baena.
The British men's number 10 ultimately paid the price for making too many unforced errors, particularly in the opening two sets.
He will also rue being unable to take any of five break points in the first game of the deciding set, having led 0-40.
"All in all, I'm happy how I performed in this match and it's something I can build on in the future for sure," said Choinski, who added he had battled a "full-blown cold and sinusitis" in the build-up.
Four other Britons - Jack Draper, Katie Boulter, Emma Raducanu and Dan Evans - play their opening matches on Tuesday.
The US Open is the first major tournament since former world number one Andy Murray retired at the Olympics last month.
Dart, who is at a career-high ranking of 78 in the world, is aiming to back up her encouraging run to the Wimbledon third round in July.
She opened up a 4-0 lead in the first set, saving three break points along the way, before Paquet eventually held serve in the fifth game.
But Dart's momentum carried her through and she broke for a third time to wrap up the opener.
The Briton suffered a brief blip in the second set, allowing her opponent to break back at 3-1 before regaining control to win the next three games and secure her spot in the second round after 76 minutes.
Dart will play 19th seed Marta Kostyuk next after the Ukrainian beat home hope McCartney Kessler.
Dart's mum is in New York with her as her coach, but the 28-year-old joked that she would not want to leave the family dog to take on the role full-time.
"I was just joking with her and said we’ve got a quite good record, maybe you want to come a bit more," Dart told BBC 5 Live.
"She would never leave our family dog. The dog is number one, definitely, out of all of us - she’s missing him dearly."
Jan Choinski was playing in the main draw of the US Open for the first time in his career
For any player ranked outside of the world's top 250, qualifying for the main draw of a major is a huge deal and Choinski will ultimately take satisfaction from his time in New York.
The prize money of £75,000 will also be useful as Germany-born Choinski looks to climb back up the rankings.
However, there will also be a sense of regret at not taking his chances to claim the second major win of his career.
Having switched allegiance to Great Britain in 2019, he reached a career-high ranking of 123rd last year but has struggled for consistency this season.
But the big server with an aggressive forehand played his way into a Grand Slam first round for the first time, having previously been given wildcards to play at Wimbledon.
Choinski - whose English mother and Polish father met while attending ballet dancing school in Germany - looked to maintain his rhythm against 55th-ranked Carballes Baena.
Trailing by two sets and a break, Choinski looked to be heading for a quick exit as Carballes Baena's doggedness and ability to put lots of balls back from the baseline led him to make a flurry of wild unforced errors.
Suddenly, midway through the third set, the British player began to find his range with winners now outweighing the mistakes.
"When I went two sets down, I still felt I was hitting quite a good ball so I just had to face the situation and make the best out of it," he said.
Demonstrating his physical and mental toughness, Choinski recovered from early breaks to win the third and fourth sets, levelling the match and forcing what had seemed an unlikely decider.
Carballes Baena left the court for an extended bathroom break, returning calmer and more composed as Choinski again became erratic on the way to losing in four hours and 31 minutes.
"There have been days where I have felt better than how I feel right now, especially fresher after that match in 32C [heat]," said Choinski.
"That puts some stress on the body, but I'll survive.
"I was struggling in the past three days to get rid of the [cold] and get my body into a position where I could go for five sets."