Serena Williams's quest for a 24th Grand Slam title came unstuck again as teenager Bianca Andreescu won her first major in a gripping US Open final.Williams, 37, could not cope with the 19-year-old Canadian's quality in a 6-3 7-5 defeat.Andreescu, seeded 15th, lost a double break in the second set before taking her third match point.To the disappointment of a stunned home crowd, Williams has now lost four successive major finals.Williams, seeded eighth, looked edgy throughout as she aimed to match Australian Margaret Court's tally of all-time major wins, handing over three of Andreescu's five breaks of serve with double faults.By contrast, Andreescu - playing in the US Open main draw for the first time - played with the confidence which has marked her out as a star in a stunning breakthrough year.The Canadian is the first teenager to win a Grand Slam since Maria Sharapova claimed the 2006 title at Flushing Meadows.She is the first teenager to win their maiden Slam since Russian Sharapova at Wimbledon in 2004.Andreescu kept her nerve to take a third match point with a forehand down the line, dropping her racquet to the ground and then, after a warm hug with Williams, falling to the court in disbelief with her arms spread out as she contemplated her achievement.After returning to her feet, she used a hastily-arranged step ladder to climb into her player's box and embrace her nearest and dearest, including parents Nicu and Maria.