"I woke up this morning like, 'Yeah, I'm beating Stefanos,'" Tiafoe said after his 6-4, 6-4, 6-3 win. "It happened. I think believing it when nobody else does is so big."
Entering the tournament ranked No. 57 and having never beaten a top-five player, Tiafoe broke Tsitsipas in the first game of the match and remained firmly in control until the final point.
And now, having taken out the top seed in his quarter and breezing through his second-round match Wednesday with a 6-4, 6-4, 6-4 win against Vasek Pospisil, the 23-year-old Tiafoe will have to rely on that same unwavering self-belief to capitalize on the upended draw.
Tiafoe never has advanced past the third round at the All England Club before, but this just might be his best chance for a deep run. He faces 25th-seeded Karen Khachanov in the third round Friday.
"It's a great opportunity now," ESPN analyst Brad Gilbert said after the match against Tsitsipas. "It's wide open."
Tiafoe gets a chance to redeem a five-set third-round loss to Khachanov at Wimbledon in 2018. No. 22 seed Dan Evans or rising American Sebastian Korda, who knocked out No. 15 seed Alex de Minaur in his opener and won his second-round match in straight sets, would potentially loom in the Round of 16.
Tiafoe wouldn't face another played seeded in the top 10 until the quarterfinals, where No. 8 Roberto Bautista Agut or No. 10 Denis Shapovalov could be waiting. Two-time reigning champion and top-seeded Novak Djokovic would be the presumed semifinal opponent coming out of the other half of the draw.
Tiafoe became just the second American man since 2010 to defeat a top-three seed at a major. He joins Sam Querrey who did it at Wimbledon in both 2016 and 2017. And if history is any indication, this might be just the start of a Cinderella run -- Querrey made the quarterfinals in 2016 and the semifinals in 2017.
Also in Tiafoe's favor: He is just over two weeks removed from his lone career title on grass. He won the Challenger event at Nottingham with a dominant 6-1, 6-3 performance in the final over Denis Kudla. It was his first tournament on grass in two years due to the pandemic, and he made the quarterfinals at Queen's Club the following week.
"It just helps my game," Tiafoe said about the grass on Monday. "It forces me to play the right way. It forces me to play super aggressive. It forces me to be off my front foot on both sides, it forces me to come to the net, because otherwise you're vulnerable ... I think these are things I'm good at."
Due to an early exit at the French Open, Tiafoe is one of the few players to play in three grass tournaments before coming to London. During a tournament in which many of his peers have had even fewer opportunities to play on grass since Wimbledon in 2019, Tiafoe's increasing assurance on the surface could be more valuable than ever.
The expectations have long been high for Tiafoe, and words like "prodigy" and "phenom" have been associated with him since he was a teenager. He became the first tennis player to sign with Jay-Z's Roc Nation Sports agency as a 17-year-old (he has since left) and was often mentioned as the next great American male star after years of searching for such a successor to Andy Roddick, who retired in 2012.
He made his major debut at the French Open in 2015 and became the youngest American man to win a title on the ATP Tour since Roddick in 2002 with a victory at Delray Beach in 2018. He made the quarterfinals at the Australian Open in 2019, and reached a career-high ranking of No. 29 soon after. He has largely struggled since, save for a fourth-round appearance at the US Open following the 2020 season restart.
But Tiafoe's play at Wimbledon, particularly his first-round upset, has reminded everyone of what all the initial hype was about.
"I think he has realized, as he has said to us and the press, that he wants to do better than he has done," ESPN analyst Patrick McEnroe said. "I think the talent and the ability is there and I think he is finding the right balance, as he has said, enjoying himself on court but being a bit more focused, a bit more professional about how he goes about his business."
Tiafoe told the media he had been exchanging messages of support with Sloane Stephens, whom he called a "great friend," before they both opened play at Wimbledon. Stephens had a tricky opponent of her own in the first round in No. 10 seed Petra Kvitova, the two-time Wimbledon champion.
Stephens won, too. And she won again, beating fellow American Kristie Ahn 7-5, 6-3.
Stephens knows about managing sky-high expectations. Following a promising junior career and a headline-generating upset win over Serena Williams at the 2013 Australian Open, she too was put on a shortlist to eventually replace Williams and her sister, Venus, as the face of American women's tennis. But she struggled with inconsistency and injuries, and her ranking had plummeted to No. 957 in July 2017. She won the US Open six weeks later and returned to the top of the tennis landscape.
Stephens had won just one match on the hard court to open the 2021 season, but made it to the fourth round on the clay courts at the French Open last month.
Tiafoe said he was inspired by Stephens' success and clearly could relate to the roller-coaster nature of her career.
"Seeing her do so well in Paris, I loved it," Tiafoe said. "She hasn't done something like that in a while. Then people forget, this girl won a Slam, made finals of another one, won a Masters series. She's one of the best girls on tour arguably, when she's fully there. There's no gimmick. This is a fact.
"You start doubting yourself. It's amazing how you win a couple matches, everything is all forgotten. I think ultimately it's understanding that it's not always peaches and cream, man. Everyone is going through certain struggles. You don't even know what these guys are going through maybe personally, whatever the case may be. It's not easy to always be playing at your top level, it's just managing the highs and lows, but always understanding that you're a baller, you're capable."
Kathleen McNamee contributed reporting.