After stunning 22-time grand slam champion Rafael Nadal to reach the US Open quarterfinal on Monday, Frances Tiafoe threw his racket on the floor and covered his face in amazement.

And with Tiafoe reaching the semifinals by beating Andrey Rublev in straight sets Wednesday, he has recorded the best grand slam result of his career -- the achievement made all the more impressive given his humble beginnings.

Tiafoe's route into tennis, after all, was in no ways traditional.

His parents met in the US after leaving Sierra Leone and had twins together, Franklin and Frances.

Their father, Constant Tiafoe, started working at the Junior Tennis Champions Center in Washington, D.C. back in 1999 and eventually moved into one of its vacant storage rooms while working around the clock.

His two boys would sometimes stay with him, sleeping on a massage table, while their mother worked night shifts as a nurse.

The unusual gateway into the sport gave Tiafoe an opportunity to start developing his skills and, after beginning to train at the facility, he didn't look back.

"Obviously, I wasn't the wealthy kid or wasn't having all the new stuff or whatever. But I was just living life. I could play tennis for free, the sport I loved," he told CNN Sport back in 2015, adding that he wouldn't change his upbringing for the world.

Driven by the work ethic of his parents, he won the prestigious Orange Bowl -- one of tennis' most prized junior tournaments -- at 15, becoming the youngest boys' singles champion in the tournament's history.

He joined a list of previous champions which included Roger Federer, Andy Roddick, Ivan Lendl, Jim Courier, John McEnroe and Bjorn Borg.

It was a sign of things to come.

Maturing on tour

Tiafoe turned professional in 2015 and started getting familiar with the rigors of the senior tour.

He broke into the world's top 100 and began to assert himself at grand slams -- reaching his first quarterfinal at the Australian Open in 2019 before losing to Nadal.

Three years on and he found himself Wednesday at another quarterfinal, only this time he felt more ready to grasp the opportunity.

"Honestly, when I first came on the scene, I wasn't ready for it mentally and mature enough," he said on court after beating Nadal. "I've been able to develop and I have a great team around me.

"I'm happy I won in front of my mom, my dad, my girlfriend and my team and to have them see what I did."

While he cements himself as a contender on the court, Tiafoe is also pursuing social justice off it.

In 2022, he told CNN Sport that the lack of diversity in the sport had made him feel like an "outsider," and he vowed to continue fighting for equality while he still had the platform to do so.

He created a protest video in 2022 to raise awareness of racial injustices after the death of George Floyd sparked protests across the world.

In collaboration with a host of Black players and coaches -- such as Serena Williams and Coco Gauff -- he posted the "Racquets down, hands up" video to his social media channels.

"Are we going to help everyone? Of course not, but I'm definitely going to help as many people as I can. That's my duty," he told CNN Sport at the time.

On the court, his match against Rublev was certainly the biggest of his career to date.

The home crowd cheered on its hero with hopes he would hit new heights in his bid to win a maiden grand slam title. And in a match where he didn't lose a service game, he didn't let them down.

After downing Rublev, the world No. 11, Tiafoe told the huge crowd, "I feel so at home at courts like this. This court is unbelievable. You guys get so far behind me, you know I want to play and want to give it my best. I always find a way somehow on this court. I always try to play great tennis and I have been. Let's enjoy this one. We got two more guys. We got two more."

His performances have even attracted the attention of some of the biggest names in sport, with NBA star LeBron James congratulating Tiafoe on making the quarterfinals.

"Thank you big bro. We got some more work to do," Tiafoe responded on Twitter.

Make no mistake, though, this is no overnight success story. It's a product of thousands of hours of work and a mentality that won't take no for an answer.

However, while the weight of a nation rests on his shoulders, Tiafoe has always just focused on making his parents proud.

"With them trying so hard, I felt like I didn't want to let them down," he told CNN Sport in 2015. "I felt like I didn't want to let the opportunities go for granted."