Serena Williams is in big trouble at the U.S. Open to unranked Italian tennis player Roberta Vinci

Serena Williams’ hopes for a calendar grand slam are in trouble as she trails 5-3 to Roberta Vinci in the semi-finals at the U.S. Open.

Serena, the world No. 1 and the heavy favourite to win the U.S. Open, looked like she would cruise to an easy win after winning the first set 6-2.

However, in the second set, Vinci, ranked 43rd in the world, stormed back to win 6-4, setting up a dramatic third set. Though Serena was up 2-0, Vinci once again stormed back, breaking Serena to go up 4-3 and then winning the next game.

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