Frances Tiafoe was described as "truly special" by his coach on Wednesday for dumping his appetite for cookies and candy in exchange for a place in the US Open semi-finals.
South African Wayne Ferreira, a former top six player who made the quarter-finals himself in New York in 1992, has been Tiafoe's coach since 2020.
He admitted, however, that when they started their collaboration, he wasn't immediately impressed.
"Food intake was terrible at the beginning. The effort on the practices and on the court wasn't good enough," Ferreira said Wednesday.
"He liked a lot of candy and chocolates and cookies. He'd eat at unusual times. He missed breakfast a lot...he was, in my opinion, not really professional enough."
Tiafoe became the first American man to make the semi-finals at the US Open since Andy Roddick in 2006 when he brushed aside Russia's Andrey Rublev in straight sets.
Roddick remains America's last home men's US Open champion from when he triumphed in 2003.
Ferreira believes the Tiafoe story is the stuff of movies.
The son of immigrants from Sierra Leone, Tiafoe is just two wins away from becoming the first African-American men's title winner in New York since Arthur Ashe in 1968.
Even if he falls in the semi-finals, Tiafoe is guaranteed to break into the world top 20 for the first time.
"It's a great story. Hopefully there will be a movie about it one day. But he has to win the Grand Slam first," added Ferreira.
"But his story is very unique, and it's a great story. He's very humble. He's a very, very nice individual. Very great heart and kind. You've got to love him. He's truly special."