The No. 7-seeded Romanian made a mere three unforced errors, a remarkably low total and 23 fewer than Williams.

Not bad for someone who has been frank about how jittery she has gotten in past big matches and began the day having lost nine of 10 matchups against Williams. But after losing each of her first three major finals, Halep now has won two straight, including at last year's French Open.

"She literally played out of her mind. Congratulations, Simona," Williams said during the trophy ceremony. "It was a little bit 'a deer in the headlights' for me."

Williams also lost in straight sets against Angelique Kerber in the Wimbledon final a year ago, and against Naomi Osaka at the US Open last September.