First, Naomi Osaka lost her cool. Then, she lost her title defense at the U.S. Open.

Teenager Leylah Fernandez pulled off arguably the biggest stunner of the tournament, rattling Osaka and storming back for a 5-7, 7-6 (2), 6-4 come-from-behind victory Friday night before an Arthur Ashe Stadium crowd, which she won over.

“From the very beginning right before the match, I knew I was able to win,” Fernandez said in an on-court TV interview, adding she was more intent on her own game than Osaka’s meltdown. “Honestly, I wasn’t focusing on Naomi. I was only focusing on myself and what I needed to do.”

Oh, Fernandez got the job done all right.

It’s the 73rd-ranked Canadian’s first victory over a top 3-ranked foe, and marks her first Round of 16 appearance at a major. And it came after either she or the moment got to Osaka: The defending champ slammed her racquet and threw it twice, all in the second set tiebreaker.

By the time Osaka took too long between points, while down 30-0 in the final game, the crowd was cheering for the underdog and jeering the champ.

“It made a huge difference. Thanks to New York. New York fans over here. They helped me get the win,” Fernandez said.

The teen won 18 of 19 on her first serve in that final set against the 23-year-old from Japan. Osaka simply was unable to read the 18-year-old’s serve or contain her emotions. Now Fernandez will move on to face Angelique Kerber, another former U.S. Open champ.

“It’ll be a battle,” Fernandez said in an on-court TV interview. “We’re just gonna have fun. I’m just going to put on a show like I did [Friday] and see what happens.”

With the first set knotted at 5-all, Fernandez dropped her serve and Osaka won 10 consecutive points.

It was 5-all in the second set, and Osaka got to a triple break point. After Fernandez took the next two points, Osaka anticipated her crosscourt shot and was there waiting, ripping a shot down the line to take the game. But this time, with Osaka serving for the match, Fernandez turned the tables.

Osaka fell behind, 40-15, and not having faced a break point all night, Fernandez earned a well-timed break.

“I guess I wanted to stay on the court a little bit linger, and I wanted to put on a show for everybody here,” Fernandez said. “One hour was not enough for me.”

That’s where the wheels really came off for Osaka.

After Osaka went down, 2-0, in the second-set tiebreak, she slammed the racquet on the court. And if she was unsettled then, she was even more visibly frustrated after falling behind 3-0.

With Fernandez serving, the teen went ahead 4-0, and Osaka threw her racquet down but didn’t get a code violation. And at 5-0, she threw it yet again.

Chair umpire Allison Hughes never even gave her a warning. It was a stark contrast to the 2018 U.S. Open final, in which Osaka beat Serena Williams thanks to the latter getting a point deducted (second violation) for slamming her racquet and a game penalty (third violation) for abusing chair ump Carlos Ramos.

Osaka eventually lost the tiebreaker, 7-2, and walked off the court, going into a small bathroom just off the court with a towel draped over her head.

After composing herself, Osaka apologized to Hughes when she eventually returned to start the third set. But Fernandez, seizing the moment and playing to the crowd, opened the set right off with a break.

Osaka hit the ball up off the court, and got a code violation warning at 40-15. Fernandez held to go up 2-0, and kept Osaka at arm’s length the rest of the set. When she held to go up 5-3, Osaka had to serve to stay in the match.

An ace gave Osaka the hold. But Fernandez built triple match point, and when Osaka sprayed her return wide, it was over.

Fernandez shrieked with joy, and thrust her fists in the air. She bounded with joy — and cruised into the Round of 16.