Serena Williams has had an amazing year, but it took a wildly unexpected turn at the U.S. Open semifinals on Friday when the No. 1 ranked Williams lost to the No. 46 ranked Italian Roberta Vinci. The stunning outcome has prompted everyone from retired Australian tennis player Rennae Stubbs to popular data analyst site FiveThirtyEight to call it the biggest upset in women’s tennis history—with good reason.

Williams was only two wins away from the first calendar-year grand slam in nearly three decades. “The Grand Slam,” for those who only watch tennis for the player grunts and ball-on-racket pops, is the nickname for the collective four major tennis tournaments: the Australian Open (January), the French Open (May/June), Wimbledon (June/July), and the U.S. Open (August/September).

It’s a rare occurrence for the same player to make it out victorious across all four in the same year, but not impossible. The last player to do so was Steffi Graf in 1988.

Williams had already taken the first three tournaments this year and headed into this most recent match with a match record of 26-0. Meanwhile, this was Vinci’s first Grand Slam semifinal.

That’s what makes this a flooring, bewildering upset. It’s daunting; probably worth a dozen 2007 New England Patriots.