The semifinal stage of the United States Open was the unexpected departure point for Serena Williams last year, a surprisingly early exit in her quest for the Grand Slam.

Perhaps the pressure of trying to win all four major tournaments in one year was the culprit, or perhaps it was merely the unexpected brilliance of Roberta Vinci on that sunny day.

A year later, there was less pressure on Williams, but there was even more brilliance from her opponent under the shimmering lights of Arthur Ashe Stadium.

Karolina Pliskova, the No. 10 seed, playing in a Grand Slam semifinal for the first time, demonstrated neither nerves nor her inexperience at this stage by defeating a hobbled Williams, 6-2, 7-6 (5), in a shocking upset at the U.S. Open on Thursday night.