When he ran into Rafa Nadal in the fourth round, it seemed like game over even before he stepped on court for the showdown with the 23-time Grand Slam champion.
Incredibly, not only did the Tiafoe topple Nadal but he now stands just two wins away from becoming the first American man since Andy Roddick in 2003 to lift the US Open trophy.
To get a chance to do that, however, he will have to navigate past the man tipped to be the next big thing in tennis - on-fire Spanish teenager Carlos Alcaraz.
"(It) put my mind at ease that Nadal is out of the way," a grinning Tiafoe told reporters following his win over Russian Andrey Rublev in the quarter-finals.
Alcaraz, however, will be no pushover. Twelve months after reaching the last eight on his Flushing Meadows debut, the 19-year-old Spaniard has returned as the third seed and showed his determination to go all the way by grinding out back-to-back five-set victories to reach the last four.
He battled into the early hours of Thursday to down Jannik Sinner in a gruelling, five-hour, 15-minute quarter-final, which kept him in the running to become the youngest ever player to climb to the top of the ATP rankings by the end of the tournament.
"It's going to be really, really tough," Alcaraz, who lost to Tiafoe in their only previous meeting, told reporters.
Like Tiafoe, this is his first Grand Slam semi-final.
"Everybody knows the level of Frances... He's playing unbelievable right now," said Alcaraz. "I feel great to be in my first semi-final in a Grand Slam. I feel better reaching semi-final here in US Open."
TENNIS SKILLS
Norwegian fifth seed Casper Ruud, who also has a link to Nadal having honed his tennis skills at the Spaniard's tennis academy in Mallorca, is the other player in contention to dethrone Daniil Medvedev as world number one.
To keep his hopes of a maiden Grand Slam title and climbing to the top of the rankings alive, the French Open finalist will need to find a way past Russian Karen Khachanov.
While Ruud is known to thrive on clay, winning three titles on the surface this year as well as finishing runner-up to Nadal at Roland Garros, he showed his hardcourt credentials by reaching the Miami final this year.
"I'm honestly a bit surprised that I made it to the semis here," said Ruud, who produced a dominant performance over Italy's Matteo Berrettini in the quarter-final.
"I have developed my hardcourt game a lot the last year or two, and I think Miami this year showed me - and I proved to myself - that I can beat good players and reach later stages in big hard court tournaments."
Khachanov had a rough run-up to New York when he suffered early exits in Washington, Montreal and Cincinnati but showed he could hold his nerve even in the toughest moments when outlasted fiery Australian Nick Kyrgios in a five-set quarter-final.
"It's like one more step forward," said Khachanov, who had never before made it past the third round at the US Open. "I'm really, really happy I could do it."
A first-time Grand Slam winner is guaranteed in Flushing Meadows, with the last four men left standing never having hoisted a major trophy.
