The 19-year-old from Mississauga, near Toronto, shocked the 23-time grand slam champion 6-3, 7-5 to win her first major trophy in her first major final.
As well as becoming the first woman to triumph at Flushing Meadows on her main draw debut, Andreescu is also the first Canadian to win a grand slam title. Prime minister Trudeau tweeted: “Congratulations. You’ve made history and made a whole country very proud.”
The Duchess of Sussex was in Williams’ box to cheer her close friend towards what was widely expected to be a record-equalling 24th grand slam title.
But the woman appearing in her first major final dealt with the occasion far better than the opponent playing in her 33rd.
Williams’ first serve deserted her, with only a wretched 44 per cent of them hitting the spot. Having been broken only three times all fortnight, she dropped serve six times in the final, including the first game and the last.
The 37-year-old is nothing if not a fighter, though.
Andreescu had a championship point at 5-1 in the second set but Williams, pictured, roared on by a vociferous crowd on Arthur Ashe, hauled herself back to 5-5. Andrescu put her fingers in her ears at one point, later admitting: “I couldn’t hear myself think”.
At the changeover at 6-5, Andreescu composed herself in the chair before holding her nerve to fashion two more match points and belting the second past the six-time champion for a remarkable victory.
“I told myself to put the goddamn ball inside the court,” she said. “I wanted to win the first point to show her that I am in it to win it.
“It was really, really loud. But I guess that’s what makes this tournament so special.
“I’m sure it’s really nice for the Americans to come on that court. I’ve heard a lot of cheering from other people for me, so I’m really happy with that.
“But it definitely wasn’t easy, especially when she started coming back in the second set. I mean, it was expected. She’s a champion. That’s what champions do. She’s done that many, many times throughout her career.
“But I just tried to stay as composed as I could. It’s hard to just block everything out, but I think I did a pretty good job at that.”
Williams was understandably frustrated with her performance, especially considering how well she played throughout the event.
“I was thinking ‘OK Serena, you didn’t miss a serve in the whole tournament, and you didn’t hit a first serve in today’,” she said. “That was obviously on my mind, like how do I play at a level like this in a final?
“Bianca obviously played well. At the same time it’s inexcusable for me to play at that level. I honestly don’t think Serena showed up. I have to kind of figure out how to get her to show up in grand slam finals.”