Botic van de Zandschulp, a Dutch tennis player, scored one of the biggest upsets of the United States Open when he stunned the four-time grand slam champion Carlos Alcaraz in the second round on Thursday. Coming from a player ranked No. 74 in the world, it may have seemed shocking, but Mr. van de Zandschulp has enjoyed the greatest successes of his career at the Open.

In 2021 he went from qualifying all the way to the quarterfinal stage, and then he turned the men’s singles draw upside down this week.

Watching from the player’s box were Mr. van de Zandschulp’s two secret weapons in New York: the Pham brothers, a pair of American former players who speak Dutch and help their childhood pal from a Dutch youth tennis program to feel at home. And they will be cheering him on again Saturday, when Mr. van de Zandschulp faces 25th-seeded Jack Draper of Britain in the third round.

But Richard and Victor Pham, both Manhattan financiers, had not been in contact with their boyhood friend for 15 years until they reunited during his first trip to the United States in 2021, when Mr. van de Zandschulp made his electrifying run to the quarterfinals. It started a tradition the three men carry on today.

“Every time I’m coming here, I have dinner with them and they come to all my matches,” Mr. van Der Zandschulp said on Friday. “And every time it is working out pretty well.”

The Phams met first Mr. van de Zandschulp, now 28, when they were boys in the Netherlands. The Pham brothers, born in Denmark to Vietnamese immigrants, began playing tennis, and played it well. Richard, now 29, hit alongside Mr. van de Zandschulp at one of the Dutch tennis federation’s training facilities when he was about 8, and eventually they were joined by his Victor, who is three years younger.