PHILADELPHIA — Skirmishing for position at the goal line ahead of a corner kick, Jozy Altidore could not believe what occurred: El Salvador defender Henry Romero bit the back of his left shoulder, then twisted his nipple.

Altidore maintained his composure — just — and then made light of the incident.

“My girl’s mad at me,” the American forward said. “She’s mad at me. She’s mad at Romero, because she’s like: ‘Only I can bite you, only I can grab your nipples.’ ”

Amid the biting, twisting and talking, the U.S. advanced to a CONCACAF Gold Cup semifinal against Costa Rica with a 2-0 win on goals by Omar Gonzalez in the 41st minute and Eric Lichaj in first-half stoppage time.

But the ugly match, interrupted by 45 fouls, will be remembered for the mark on Altidore’s shoulder left by the teeth of the 25-year-old defender. Gonzalez said after the game he was bitten on the back of his left shoulder, by Darwin Cerin in the 81st minute.

“I was a bit shocked,” Gonzalez said.

U.S. coach Bruce Arena said he wasn’t surprised.

“There’s a history of that in our sport,” he said. Uruguay’s Luis Suarez was given a four-month suspension for biting Italy’s Giorgio Chiellini during the 2014 World Cup.

A furious Altidore shoved Romero, then grimaced and fell to the ground but stayed in the game.

“It’s never happened before, but in CONCACAF, it never ceases to amaze me,” Altidore said. “You got to keep your cool. … I shouldn’t be saying these things happen, but they do.”

Canadian referee Drew Fischer, a Major League Soccer regular, did not penalize the incident, which occurred in the 57th minute.

“I can’t fault the referee,” Arena said. “Those things are not easy to see on the field.”

CONCACAF’s disciplinary committee has leeway to impose punishment.

“These things always happen in football,” El Salvador coach Eduardo Lara said through a translator, though it was unclear he knew bites had occurred.

The Americans, who started five veterans added for the tournament’s knockout phase, overcame a shaky defense and poor passing. They face Costa Rica on Saturday in Arlington, Texas.