A snow storm blanketed Fetzer Field and made the game a battle of which team could stay on its feet and make solid contact with the ball.
The Tar Heels didn't even use their feet to score. They headed in three goals to erase a 2-0 second-half deficit and earn an overtime victory Sunday.
UNC will play Rhode Island in the second round.
"You can judge for yourself what it was like out there to play in those conditions," UNC coach Elmar Bolowich said.
"It is definitely a different game. It came down to just simply in the end to guts and battle and fight and not giving up anything."
The slippery conditions limited UNC's usual speed attack led by forwards Chris Carrieri and Caleb Norkus.
Instead, it was the Tar Heels' height that was the major factor in the victory. UNC headed in all three goals off reset plays that resulted from fouls or corner kicks.
"North Carolina has a very big team," William & Mary coach Al Albert said. "It was just a little bit too much size for us to handle."
Carrieri did have an impact in the game-winning goal in the fourth minute of the overtime. After his corner kick got blocked out of the box, he recovered the loose ball and sent another cross into the box.