The Grand Slam seemed there for the taking. The Grand Slam will have to wait.

Two victories from joining the most prestigious club in tennis and with her path clear of all the most evident thorns and threats, Serena Williams ran into an unlikely roadblock in the form of Roberta Vinci.

In one of the biggest surprises in tennis history, Vinci, an unseeded Italian veteran playing in her first Grand Slam semifinal, defeated Williams, 2-6, 6-4, 6-4, at the United States Open on Friday.

“The best moment of my life,” said Vinci, who embraced the occasion thoroughly, cupping her hand to her ear and exhorting the crowd after winning a spectacular rally, and crying in her chair after the upset.

Vinci, who had never won a set against Williams in four previous matches, also apologized.

“For the American people, for Serena, for the Grand Slam and everything,” she said. “But today is my day. Sorry, guys.”