He never had been past the third round in his previous 15 majors under his run to this year's French Open quarterfinals, when he was forced to retire with an abdominal injury in the second set against eventual champ Nadal.
Shapovalov, 18, became the youngest player since Michael Chang in 1989 to advance to the fourth round of the U.S. Open when British opponent Kyle Edmund retired from their third-round match on Friday.
The teenage Canadian qualifier, who is predicted to become a future grand slam champion, succumbed to the Spanish 12th seed 6-7 (2) 6-7 (4) 6-7 (3).
Still struggling to find his stroke, Shapovalov turned to the crowd and shouted with joy after an overhand smash cut Carreno Busta's lead to 5-4 and give the young Canadian serve. After a huge return set up a break point, the Spaniard netted his backhand to surrender the break. Due to lack of experience, however, it wasn't long before the experienced Spaniard held serve and found himself back on serve. All of a sudden, the teen fell apart under pressure, missing a series of forehands to set up double break point. I continued fighting all the time.
After surviving the first twelve games by the skin of his teeth, Carreno Busta took full advantage in the tiebreak. As the new "it" man in both world tennis and Canadian sporting circles, Shapovalov is seeing a considerable boost in notoriety, even among the type of people that aren't usually associated with tennis.
After a couple of errors from Shapovalov, Carreno Busta broke back with a stunning forehand on the run. However, Carreno Busta broke back to win three straight games and tie it 3-3.
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Denis Shapovalov heard opportunity knocking.
Just as the Canadian failed to serve out the opening set, Carreno Busta would follow suit in the second.
Serving for the set at 5-2, Shapovalov blinked. Once again, the set required a tiebreak, but that's where the role-reversal stopped as it was again the Spaniard taking control. In the second set, I was 5-3 on serve and I lost it.
Shapovalov very almost let the set get away from him immediately, being forced to save a pair of break points in the opening game. Carreno Busta stretched the lead to 6-1 then converted his second set point stealing a 58-minute comeback set. Pablo consolidated the mini-break to lead 3-0 in points. They have been really carrying me.
Denis Shapovalov of Canada in action during his fourth round match against Pablo Carreno Busta of Spain on Day Seven of the 2017 US Open at the USTA Billie Jean King National Tennis Center on September 3, 2017 in the Flushing neighborhood of the Queens borough of New York City. The lower half of the Men's Draw is deplete of many top seeds while the first and third seeds march on for a possible semifinal clash. With that comes demands on and off the court that Shapovalov will have to learn to handle if he's going to continue to navigate the intense gaze of stardom in one of the world's most popular sports.
"Honestly, it was so much fun to be part of that atmosphere and the match and this whole two weeks", Shapovalov said. The Spaniard remained ideal on serve and added another minibreak when the Canadian sent a backhand long, stretching the lead to 4-0 before setting up five match points at 6-1. Two more unforced errors by Shapovalov made it 5-5, with Carreno Busta's consistency wearing down the emotional teenager.