Williams was denied a record-equaling 24th Slam title as the 19-year-old Andreescu on Saturday became the youngest Grand Slam winner since Maria Sharapova at the 2006 US Open.
While it was the fourth successive defeat in the finals for Willams, Andreescu tied Monica Seles, who lifted the 1990 Roland Garros trophy in her fourth major, for the record as the fastest player to grab a first Slam crown in the Open era.
The morning showed the day as Williams by conceded her opening service game with successive double faults at Arthur Ashe Stadium.Andreescu did not let the gifts go in vain as she capitalised on them and took the first set as Williams handed her the first with another double fault.
In the second set, Andreescu, who had failed to reach the main draw at Flushing Meadows in the previous two years, surged 2-0 ahead.
Williams fought back from being 5-1 down to level 5-5 by hitting 14 of 17 points in a pulsating contest.
The Canadian then held for 6-5 and Williams erred on a backhand to give Andreescu two more shots at the title. Williams hit an ace to save the first but Andreescu secured victory on the next with a crisp forehand return.
( With inputs from IANS )