Which player ended Serena Williams's bid for a calendar-year Grand Slam in 2015?

Roberta Vinci ended Serena Williams’s bid for a calendar-year Grand Slam in the 2015 US Open semifinals.

Williams entered the tournament having already won the Australian Open, French Open and Wimbledon. A victory in New York would have completed the first calendar-year Grand Slam in women’s singles since Steffi Graf achieved it in 1988.

Vinci, an unseeded Italian, overturned the match after losing the opening set. She defeated Williams 2–6, 6–4, 6–4, producing one of the biggest upsets in tennis history. Vinci had lost all four of her previous matches against Williams, making the result even more unexpected.

Vinci then faced fellow Italian Flavia Pennetta in the final. Pennetta won 7–6(7–4), 6–2, so Vinci stopped Williams’s Grand Slam pursuit without winning the tournament herself. That distinction is why Pennetta is the 2015 US Open champion, but Vinci is the player credited with ending Williams’s calendar-year bid.