No. 7 Marin Cilic and Alex de Minaur battle for over four hours, with Cilic coming out on top in the fifth set to advance at the US Open. (1:32)

NEW YORK -- Everyone is impressed. Tennis, its fans and even Alex de Minaur's opponents.

After four hours of grueling baseline tennis, when No. 6 seed Marin Cilic smashed a crosscourt forehand at match point on Sunday, the Australian Alex de Minaur still didn't give up. He ran from the opposite end of the court at 2:22 a.m. local time, trying to return the ball and stay in the match. He had worked so hard, having won the first two sets and clawing his way back from 5-2 to 5-5 in the fifth set.

But victory wasn't meant to be.

He stretched his hand and tried to get to the ball, and when he couldn't, he finally let go, a defeated look across his face. The crowd, even in that wee hour, cheered loudly, having appreciated the incredible match.

"He showed an incredible spirit. It was just an incredible comeback from Alex: 5-2 up for me and love-40. Throughout the match he was playing great tennis, what separated us was a couple points, nothing else." No. 6 seeded Marin Cilic on Alex de Minaur

De Minaur has earned respect by way of his fight and never-give-up approach.

"He showed an incredible spirit," Cilic said. "It was just an incredible comeback from Alex: 5-2 up for me and love-40. Throughout the match he was playing great tennis, what separated us was a couple points, nothing else."

It took five tough sets for the No.-6 seeded player, Marin Cilic, to defeat Alex de Minaur at the US Open. AP Photo/Mark Lennihan

At 19, Alex de Minaur has beaten some of top-ranked players -- Milos Raonic, Fernando Verdasco and Feliciano Lopez -- just this year. And late on Saturday night, he came close to beating another. He saved six match points in the fifth set and at 5-5 after more than three and a half hours, it still looked like it could go either way.

"Right now, I'm completely devastated, but it's a great learning experience for me," de Minaur said during his postmatch press conference. "I did everything in my power to try and get back, every ounce of energy to try and push. I managed to come back but he was too good in the end."

Cilic went on to call it one of the best matches of his career.

"This kid is unbelievable. Never say die. It is great to see," Australian coach, Darren Cahill, said during the ESPN broadcast.

My son finally wants to play tennis because of Alex De Minaur. What a legend. Huge fighter.... - Kate Sheahan (@KateSheahan1) September 2, 2018

I hope Alex de Minaur becomes the tennis player that Australia has wanted Kyrgios/Tomic/Kokkinakis to become. #USOpen - Annanya (@annanyajohari) September 2, 2018

Born in Sydney to a Spanish mother and Uruguayan father, de Minaur is both multicultural and a polyglot -- he speaks English, French and Spanish. He lived the early years of his life in Australia before moving to Alicante, Spain. He moved back to Australia in 2012 and splits his time between Spain and Australia. As soon as he moved, he decided he was going to represent Australia in Tennis. He's has said he feels felt a strong bond towards Australia, and that's the flag he'd like to see next to his name at tennis tournaments.

Despite this loss, the Australian is having a career-changing year. He reached the third round at Wimbledon and US Open, beating Marco Cecchinato and Pierre-Hugues Herbert at Wimbledon and America's Francis Tiafoe here in Flushing Meadows in a four-set upset.

To put in perspective how much he has achieved at just 19 years of age -- he played in the semifinals of two consecutive ATP tournaments (Brisbane and Sydney). In the process, he became the youngest player to do so since Rafael Nadal in 2005.

At the 2018 Citi Open in Washington, he finished the tournament as the runner-up, falling to No. 4 seed Alexander Zverev. But that was not before he defeated a slew of seeded players -- No. 11 Steve Johnson, Australian Open semifinalist and No. 8 seed, Chung Hyeon, and Russia's Andrey Rublev. Saving four match points in the second-set tiebreaker, the Australian won the next six points to take the set -- and the match -- 8-6.

"We have a lot to look forward to over the next 10 years. There is a lot of Lleyton Hewitt in the way Alex goes about his tennis, with his fight and his spirit," Cahill said.

That "we" is everyone. Australia and all of tennis.