Bianca Andreescu is the U.S. Open champion.

The Mississauga, Ont., teenager downed tennis great Serena Williams 6-3, 7-5 on Saturday to become the first Canadian to win a Grand Slam singles title.

The 19-year-old used her powerful forehand — and was helped by an uncharacteristically bad service game from the veteran Williams — for her third tournament win of the season, and biggest by far of her career, in her first U.S. Open main draw appearance.

Andreescu converted on her third championship point of the match, breaking Williams with a forehand return before ecstatically sprawling belly-up on the court.

Watch highlights from Bianca Andreescu's U.S. Open victory:

Bianca Andreescu becomes the first Canadian in history to win a Grand Slam singles title with her straight-set victory over Serena Williams. 1:45

Then she got up, climbed a ladder to get to her players' box and embraced coach Sylvain Bruneau and her mom and dad.

Seeded No. 15 at the major, Andreescu will reach a career high No. 5 when the new WTA rankings are released Monday. She also earned $3.85 million US for the victory and improved to 8-0 against top 10 opponents — all this year.

She the North indeed

Andreescu put her hands over her mouth in awe after the match when the on-court interviewer mentioned that she was the first Canadian to win a major.

"It's so hard to explain in words but I'm beyond grateful," Andreescu told him. "I worked really hard for this moment.

"This year has been a dream come true and being able to play on this stage against Serena, a true legend in this sport, is amazing."

Andreescu's father, teary-eyed in the crowd, was shown filming the post-match interview and trophy presentation on his cell phone.

While there was a loud applause from the crowd when Andreescu was introduced before the match, the ovation for Williams was deafening.

Andreescu said she had to fight off the crowd noise at times, especially when she saw her 5-1 lead evaporate in the second set.

"I know you guys wanted Serena to win so I'm so sorry," she said on the court.

It wasn't a pretty match for Williams, who struggled on her serve throughout, with her first-serve percentage dipping to 25 at one point in the second set.

But the 23-time Grand Slam champion showed fight late, staving off one championship point and breaking Andreescu two straight times to tie the set 5-5 and send the crowd of 26,191 at Arthur Ashe Stadium into a frenzy.

Watch Championship Point:

Bianca Andreescu defeated Serena Williams 6-3, 7-5 to capture the women's singles U.S. Open title at Arthur Ashe Stadium. 1:09

The No. 8-seeded Williams won her first U.S. Open title here 20 years ago — before Andreescu was born.

Saturday's match lasted 1 hour 40 minutes, about 80 minutes longer than their last meeting, at the Rogers Cup final in Toronto last month.

Andreescu won that title when Williams retired with a back injury. The Canadian was leading 3-1.

Watch as Andreescu consoles Williams after Rogers Cup win:

Bianca Andreescu and Serena Williams shared an emotional embrace after Williams was forced to retire from the Rogers Cup final due to injury. Andreescu is the first Canadian woman to win the Rogers Cup since Faye Urban in 1969. 2:14

Andreescu, who began the year ranked No. 152, headed into the U.S. Open after a stellar season that included her first two titles.

She has been one of the best Canadian sports stories of the year, garnering praise from the likes of Wayne Gretzky, Sidney Crosby, Steve Nash, and Kia Nurse. Even Billie Jean King, whose name adorns the tennis centre where the U.S. Open is played, has tweeted about Andreescu multiple times.

All this for a player who didn't even make it out of the qualifying round at Flushing Meadows in 2018.

A year later, Andreescu rode her aggressive style to a championship, downing Katie Volynets, Kirsten Flipkens, No. 19-seed Caroline Wozniacki, Taylor Townsend, No. 25-seed Elise Merten and the No. 13 Belinda Bencic to become the first teenager to win the U.S. Open since Maria Sharapova in 2006.

She attacked from the beginning Saturday, breaking Williams twice in the first set — in the first game and on a double fault to end the set — while facing just one break point.

After Andreescu won, she pumped her fists and congratulated Williams on a well-played match. Then she lay on the court and covering her face as she processed the win. (Danielle Parhizkaran/USA TODAY Sports)

Williams double-faulted again to give Andreescu another break and a 2-0 lead to start the second set. The American responded by breaking Andreescu for the first time, but then lost her next serve as the Canadian built a 5-1 advantage.

Williams, whose disappointment was etched on her face during parts of the match, remains one major victory short of tying Margaret Court's 24 titles.

She is also still looking for her first championship — major or otherwise — after returning from the birth of her daughter. Williams had won the 2017 Australian Open while pregnant and returned to the WTA tour at Indian Wells in March of 2018.

Williams had looked otherworldly throughout the U.S. Open, carving up opponents starting with Sharapova in the first round. She also notched the quickest victory of the tournament, a 44-minute one-sided quarterfinal win over China's Wang Qiang.

Andreescu was the first Canadian woman to play in a Grand Slam final since Eugenie Bouchard competed for the Wimbledon title in 2014.

#SheTheNorth was trending on Twitter during and after the match. Andreescu's achievement was recognized by Prime Minister Justin Trudeau, among others.

Congratulations <a href="https://twitter.com/Bandreescu_?ref_src=twsrc%5Etfw">@Bandreescu_</a>! 🇨🇦 You’ve made history and made a whole country very proud. <a href="https://twitter.com/hashtag/SheTheNorth?src=hash&ref_src=twsrc%5Etfw">#SheTheNorth</a> <a href="https://t.co/W98v1lUN9o">https://t.co/W98v1lUN9o</a> —@JustinTrudeau

The 2019 NBA Champion Toronto Raptors, who use 'We The North' as their team hashtag, also congratulated Andreescu with a tweet that read 'She The Champ' accompanied by a photo of an Andreescu Raptors jersey laying next to the Larry O'Brien trophy.