Daniil Medvedev of Russia won his first Grand Slam championship in a one-sided US Open final, ruining Novak Djokovic's ambitions of completing a calendar sweep of the majors.
Second seed Medvedev, 25, had lost his last two Grand Slam finals but remained composed throughout his 6-4 6-4 6-4 victory.
Djokovic, the world number one, had previously won the Australian Open, French Open, and Wimbledon this year.
The 34-year-old Serb was vying for a record 21st Grand Slam men's title.
Victory would have put him ahead of rivals Roger Federer and Rafael Nadal in the race for the most men's majors, but it was the possible calendar Grand Slam that Djokovic had described as his "biggest achievement" in his career.
Djokovic was attempting to become only the sixth player in history to win a calendar Slam, and the first man to do it since Rod Laver in 1969.
Djokovic was somber throughout, may be burdened by the weight of history, making a high number of unusual mistakes, and even tearful at the finish.
He also appeared to be way short of his usual physical capabilities.
It would be incorrect to attribute Medvedev's success only to those factors.
The Russian played smartly and comfortably, forcing errors from his opponent with strong serving, dominating from the baseline, and finishing points with precise winners to revenge a straight-set loss to Djokovic in the Australian Open final in February.
There was a small stutter as he attempted to serve for the title - three double faults allowed Djokovic to grab back a break for 5-3 - but Medvedev calmed down to take his third championship point.
Medvedev burst out in a big smile after exchanging a heartfelt hug with Djokovic at the net, before leaning back in his chair to consider finally capturing the first Grand Slam championship that he has long threatened.
"I want to thank my team, those who are here and watching. My parents, my family, my sisters. Some of my friends are here too," Medvedev said after the match.
The 25-year-old dropped only one set in the tournament.
"Thank you guys because it is not an easy journey to win a Slam. I'm really thankful to you for helping me throughout this journey," he thanked his family and friends.
"If there is anyone who deserves a Grand Slam title right now, it is you. Well done. There will be many more titles to come, I'm sure," Djokovic appreciated Medvedev for his spectacular performance.
Third attempt at the Grand Slam for Medvedev
Medvedev made an emotional figure following the men's singles final in New York two years ago.
It was the Russian's first Grand Slam final, and after an incredible comeback from two sets down, he finally fell to Nadal in an epic five-setter.
After being captivated by a video montage on the Ashe television screens depicting all of Nadal's accomplishments - then totaling 19 Grand Slam championships - the Russian subsequently stated that he intended to utilize it as extra inspiration in his own quest to win the sport's most prestigious awards.
He has finally accomplished that.
Medvedev was overwhelmed by Djokovic's tremendous pressure in Melbourne on his second effort, and his game broke down.
It was a total role flip this time.
Medvedev broke in the first game of the match, and while Djokovic settled down in his service games for the rest of the set, he couldn't affect Medvedev's serve.
Even the finest returner in the men's game had no answers when they were fired down with pace and precision.
Medvedev only let Djokovic earn three receiving points in the first set, winning 100% of his first-serve points while Djokovic made only 27% of his returns.
Djokovic had lost the first set in three of his previous four matches, including victories against German fourth seed Alexander Zverev and Italian sixth seed Matteo Berrettini, and he had five break chances at the start of the second set.
But he couldn't convert either of them as Medvedev fought to hold both times, leaving Djokovic smacking his thighs in frustration.
The Russian broke for 3-2 in the second set, then raced to a 4-0 lead in the third before securing victory after a minor wobble.
Following two double faults on his first two championship points, Medvedev calmed himself to deliver the first serve on the third.
Djokovic was unable to return it over the net, and world number two Medvedev dropped to the ground in a 'FIFA'-inspired celebration.
New York was never this welcoming to me: Djokovic
While the loss of the chance to make further history would be difficult for Djokovic to swallow, there was some solace in the raucous ovation he received from a New York audience with whom he has not always had the best of ties.
The Arthur Ashe Stadium has been a difficult environment for Djokovic in the past, notably in 2015 final versus Federer, when spectators cheered on the Serb's misses.
There was a lot of support for the Serbian 20-time Grand Slam winner from the start of the final against Medvedev until he walked off the court.
After a comfortable hold for 5-4 in the third set, the fans erupted in favor of Djokovic, causing him to become emotional in his chair.
He laughed at first, then beat his chest as the sounds got louder. When a result, an exhausted Djokovic sobbed under his towel on his chair and was still sobbing as he stood on the baseline at the start of the following game.
"Even though I haven't won the match I am the happiest man alive because you guys made me very special on the court," Djokovic said about the New York Crowd.
"You guys touched my soul. I've never felt like this in New York. Thank you for your support. I love you."