Canadian teenager Bianca Andreescu broke Serena Williams' serve twice to take the first set of the US Open final 6-3.
Williams, who's vying for a record-tying 24th Grand Slam title, lost the set on her third double-fault of the match.
Andreescu, 19, showed no sign of nerves despite making her first-ever appearance in a Grand Slam final. She converted a break point in the opening game and pushed Williams in a wild seventh game that lasted 10 minutes.
Williams saved five break points to give herself a chance, but her service game let her down. She made just 50% of her first serves compared to Andreescu's 84%.
