Andreescu's father, teary-eyed in the crowd, was shown filming the post-match interview and trophy presentation on his cell phone.
It was the only break she needed as Andreescu, rock solid on her own serve and with a potent mix of power and spin from the baseline, controlled the first set.
"I just wanted to make sure so I didn't look like an idiot", Andreescu said.
I tried to just do that throughout the whole match, to just keep my nerves in place.
Here are more details on her win and how much prize money Bianca Andreescu will get for the victory.
Serena Williams lost her temper and then lost the match when she played for last year's U.S. Open championship.
Maybe someday, in the distant future, Serena Williams will look back and be proud of herself for making it to the final at four of the first seven Grand Slam tournaments she played in after having a baby.
This is the second year in a row that Williams has lost in the U.S. Open final. At the 2019 Rogers Cup in Toronto, Andreescu defeated Williams 3-1 before Williams had to pull out with a back injury. "For it to become a reality is just so insane", she said.
Andreescu, Williams' opponent on Saturday, wasn't even born by the time Serena secured her first U.S. Open crown in 1999.
Both players held serve comfortably in the next few games before Williams finally earned a break point in the eighth game.
Williams saved a championship point in the second set - a moment met with rapturous applause and noise around Arthur Ashe Stadium.
"She will, she will keep trying next year, no doubt about it but this will sting more than the other ones". However she was then beaten by Andreescu in the next two games.
"Also, I mean, it's part of life going through situations like that", Andreescu said. I think he'll shine when the season starts.
"It was really, really loud".
A forehand victor earned the youngster a set point, and Williams duly her gifted her the opener with another double fault. Williams struggled with unforced errors, and finished the match with 33 unforced errors to 33 winners.
Each time, she fell short of victory. "I have to do better, '" Williams said.
I think she's a great girl. It's just taking it.
Williams: "I don't know how to answer that". "The fans started cheering so hard and it made me feel better and fight a bit more". I'm not necessarily chasing a record. "It's hard to know that you could do better". "I'm still doing what I can do".
This loss will sting, but she's already on to the next - Grand Slam title No. 24 is inevitable.
She has not won a title of any sort since the Australian Open in January 2017. She is the first woman since Monica Seles in 1990 to lift the trophy in only her fourth Grand Slam tournament.
She couldn't even get into main draw of the U.S. Open past year after losing in qualifying.
Andreescu won at Indian Wells and in the Rogers Cup, too, and reached a career-high number five in the WTA rankings on Monday following her NY success.
"Yeah, it was incredible tennis". On Saturday, she has the chance of ending her breakout season on the kind of high she wouldn't have dared imagine 12 months ago. She's done that many, many times throughout her career.
"I was so clueless out there", she recalled with a laugh.