In doing so the 19-year-old Canadian denied Williams the chance to equal Margaret Court's all-time record of 24 majors.
Here, PA looks back at day 13 and ahead to day 14 at the US Open.
Shot of the day
— US Open Tennis (@usopen) September 7, 2019
Andreescu will remember her third championship point for the rest of her life.
Stat of the day
Andreescu is the first winner of a grand slam to have been born in the 2000s. There has yet to be a men's champion born in the 1990s.
Tweet of the day
From the Canadian prime minister, no less.
Quote of the day
Jamie Murray ensured some British success at the US Open by defending the mixed doubles title with America's Bethanie Mattek-Sands. The 33-year-old claimed his seventh grand slam doubles title, and fifth in mixed, with a 6-2 6-3 victory over top seeds Chan Hao-ching and Michael Venus. In the latest wheelchair quad singles round-robin Andy Lapthorne beat American Bryan Berten 6-1 6-3.
Mattek-Sands did not actually know it was championship point when she smacked a winning return. "Actually I was really far off. I thought it was 3-2 in games," she admitted afterward.
Rafa's after Roger
Rafael Nadal will bid to move to within one grand slam title of Roger Federer's record of 20 in Sunday's final. Standing in his way is Russian Daniil Medvedev, the fifth seed who has cramped up, cursed and goaded the crowd on his way to a first major final.
Order of play
Arthur Ashe Stadium:
Daniil Medvedev v Rafael Nadal (2100BST)