Novak Djokovic is three wins away from a calendar Grand Slam.

Djokovic advanced to the quarterfinals of the 2021 U.S. Open with a 1-6, 6-3, 6-2, 6-2 victory over Jenson Brooksby in Monday's fourth-round matchup at Arthur Ashe Stadium. It was anything but easy with the underdog putting up an impressive fight as he played off the crowd, but the 20-time major champion overcame a slow start and prevailed.

The first-ever meeting between the two was something of a David-against-Goliath showdown.

After all, Djokovic is on the shortlist of the best tennis players of all time and can strengthen any argument in his favor with the calendar Grand Slam. Accomplishing it would make him the first male player of the Open Era to claim it since Rod Laver did in 1969.

It would also put him ahead of Roger Federer and Rafael Nadal with 21 major titles.

As for Brooksby, he became the youngest American to reach the fourth round of the U.S. Open since a 20-year-old Andy Roddick did so in 2002 and impressed with a five-set victory over Aslan Karatsev in the third round.

"It will be a great challenge," Brooksby told reporters when previewing Monday's match. "One of the toughest there can be. But I'm really believing in myself with what I'm showing out there so far. I got a great team around me to help me recover. ... [It] will be really exciting."

It looked like anything but a "great challenge" in the early going, as the American cruised to a 6-1 opening-set victory. The crowd quickly got behind the underdog, and he even gestured for the spectators to make more noise after closing the stunning set.

Djokovic dropped a set in the first round to Holger Rune and the third round to Kei Nishikori, but this felt different. Brooksby was hitting winners, battling through long rallies and capitalizing on unforced errors from the all-time great.

On paper, a sense of normalcy returned in the second set with the favorite winning 6-3. However, it was anything but normal thanks to an incredible fifth game that went back-and-forth on deuce and lasted longer than 18 minutes.

That Brooksby won the game and earned a break even though he eventually lost the set further underscored the reality he was anything but intimidated by the moment.

At some point, though, it was just a matter of time before Djokovic's immense talent level broke through over the course of such a long and grueling match. Perhaps fatigue was an issue for Brooksby after the marathon second set, but the third one took a turn for the worse for the challenger as the top seed won 6-2 without much of a challenge.

It was largely a formality from there, especially when Brooksby made two critical unforced errors on his final service game of the fourth set to fall two breaks behind.

Djokovic's physicality was simply overwhelming as the match continued, and the 20-year-old couldn't match it despite impressing in the early going. Brooksby certainly made a name for himself on a national stage, but one of the best ever eventually dialed in and took over.

Djokovic will now face Matteo Berrettini in the quarterfinals.