Bill Ackman, the billionaire financier, has succeeded at pretty much everything he’s done, professionally and otherwise. He built his hedge-fund, Pershing Square Capital Management, into a winner. He backed President Trump’s third campaign — and second term. And he’s a prolific poster on social media, regularly plopping long treatises on X, which helped him become internet-famous.

Tennis, however, is different.

That was the lesson seemingly taught on Wednesday afternoon when Mr. Ackman, 59, and his doubles partner, the retired professional Jack Sock, lost in straight sets to a pair of journeymen in front of a sweaty and well-heeled crowd at the International Tennis Hall of Fame in Newport, R.I., a famed summer retreat for Gilded Age industrialists.

The loss came in the Hall of Fame Open, an official tournament of the lower-level ATP Challenger tour, which effectively made Mr. Ackman a tennis pro, if only for a day. It fulfilled a lifelong ambition — and, more than likely, he said, ended his career.

“I feel like maybe it’s one and done,” Mr. Ackman said, in the wake of his and Mr. Sock’s 6-1, 7-5 defeat, noting he wanted to support younger players getting their tournament slots — and shots. “But I figured one, in my life, that seemed fair.”

Some spectators, however, had different interpretations.

“Another mega-billionaire living out his own private dream,” said Lydia Chambers, a tennis fan from New Vernon, N.J., who was watching the match. “I hope he’s making a huge donation.”