Frances Tiafoe has beaten three higher-ranked players to reach the quarterfinals of the Australian Open. (Lynn Bo Bo/EPA-EFE/Shutterstock)

It was the highlight of Frances Tiafoe’s teenage years to be invited to practice with Rafael Nadal during the 2014 French Open.

Then 16, Tiafoe was a widely heralded tennis phenom and top seed in that year’s junior boys’ event at Roland Garros, while the 28-year-old Nadal was firmly ensconced as the “King of Clay,” with eight French Open championships and a ninth in his sights.

After their grueling, 90-minute hitting session on a side court at Roland Garros ended, Tiafoe got his photo taken with Nadal but trudged off the court with a far more valuable memento: a firsthand look at the total commitment of a top player such as Nadal.

What floored Tiafoe wasn’t simply the heaviness of Nadal’s topspin blasts; it was how hard the world’s best clay-court player practiced, taking nothing for granted, drilling every shot in his arsenal at peak intensity, over and over and over again.

“I remember that like it was yesterday,” Tiafoe said in a telephone interview from Melbourne, Australia, the morning after his 7-5, 7-6 (8-6), 6-7 (7-1), 7-5 upset of Grigor Dimitrov clinched a spot in the quarterfinals of the Australian Open — his first Grand Slam quarterfinal — where he will face Nadal on Tuesday for the first time in his career. The match is expected to start between 4 and 5 a.m. Eastern time.

“It’s crazy that we’re going to play each other,” Tiafoe mused. “It was one of the biggest moments of my career, to hit with him at the French Open. It should be called the Nadal Open, he’s won it so many times. But it’s going to be different [Tuesday]. I’m not that same kid.”

Now 21, Tiafoe will step into Rod Laver Arena with sky-high confidence, having toppled three higher-ranked players, including the No. 5 and No. 20 seeds, to reach the final eight.

“I’m high on life right now,” Tiafoe said. “When I’m out there, I feel like I can do everything. I feel like everything makes sense. Like the game is going so slow.”

Regardless of the outcome in the quarterfinal, Tiafoe, a Hyattsville, Md., native who learned to play at the Junior Tennis Champions Center in College Park, will vault from 39th in the world to at least a career-high ranking of 29th. With a victory over the second-seeded Nadal, who is seeking his 18th Grand Slam championship, he would climb into the top 25.

World rankings are just one measure of how much Tiafoe has grown since that early-morning hitting session in 2014.

He is an inch taller than he was at 16, now 6-foot-2, and carries at least 35 more pounds of muscle mass. That has translated to a stronger serve and more power in an already wicked forehand.

He is also a more aggressive tactician, in the approving eye of Brad Gilbert, a former professional player and coach turned ESPN analyst who has followed Tiafoe’s development since Tiafoe won the Orange Bowl junior title at 15. Gilbert also notes the seasoning of big-match play.

“The more matches you play at this level, you become older than your age,” Gilbert said in a telephone interview from Melbourne.

As a junior player, Tiafoe tended to let negative emotions derail his focus in tough patches. He would sulk and lose focus. These days, he is more composed on court. Yet he hasn’t lost the passion and exuberance that have made “Big Foe” a fan favorite, whether shedding euphoric tears or channeling his idol LeBron James in muscle-flexing victory celebrations.

Said Tiafoe: “Obviously, maturity helps, knowing that you definitely have to make every day a good day. If not, somebody else is out there putting in the work. I’m still taking baby steps toward that. I’m not perfect. But I hold myself to a high standard every day. I maximize every day.”

Developing tennis champions is a painstaking, inexact process that requires technical expertise, tactical savvy, athletic and mental strength and profound self-belief. All but the last piece can be coached. Belief must come from within.

In the experience of Ray Benton, a pioneering tennis agent and promoter before becoming CEO of the Junior Tennis Champions Center, self-belief is what separates champions from equally talented players who train just as hard.

“When Jimmy Connors was 18 years old, he felt he was ready to beat the best pros in the world,” said Benton, whose clients included Connors. “Mats Wilander, at 17, wasn’t cocky going into the French Open, but he knew he belonged there.”

When Benton looks at Tiafoe, whom he has known since the player was a child, he sees a phenomenally talented player who is starting to believe he belongs among the best.

“Frances has been told he belongs, but he’s a normal, well-adjusted kid, so it takes a while to believe it,” Benton said. “He is just starting to feel comfortable at this level. Frances is still very young. He has only reached about 50 percent of his potential.”

In reaching his first Grand Slam quarterfinal, Tiafoe, along with 20-year-old Stefanos Tsitsipas of Greece, has assumed a leading role in a story line that tennis has trumpeted for years: the emergence of the next generation of players who will supplant Roger Federer, Nadal and Novak Djokovic.

He is also part of a new generation of Americans asserting themselves on the sport’s global stage. Joining Serena Williams in reaching the women’s final eight in Australia is surprise quarterfinalist Danielle Collins, a two-time NCAA champion at Virginia who had never won a Grand Slam match.

But no player has the unique narrative of Tiafoe, whose parents immigrated to the Washington area from war-torn Sierra Leone in the 1990s. His father, Constant, got a job in construction, helping build College Park’s JTCC, and was then named maintenance director of the complex. It was while minding his twin boys at work while his wife held down shifts as a nurse that Constant put tennis rackets in the 4-year-olds’ hands to keep them occupied.

Ever since he can remember, Frances wanted to be a tennis pro. Last year, he bought his mother a home in Maryland with his winnings; his father is settled in an apartment in Florida.

Having improved his family’s situation and made both parents proud, Tiafoe has more to achieve in tennis. He is striving for greatness.

“I’m going for the big moments — I want the big moments,” Tiafoe said on the eve of the quarterfinal. “I just tell myself, ‘Remember the work you did, the things you went through. And keep going.’ ”