Greenberg, who signed a one-day contract for Tuesday's appearance with the Miami Marlins, took strike one, hacked at a pitch off the plate, and went down swinging on a pitch up and out of the zone.
But he went to the dugout with a smile on his face, receiving hugs and handshakes from teammates and Marlins Manager Ozzie Guillen.
Greenburg's second-chance story began in 2005, when in his major-league debut as a member of the Chicago Cubs he was drilled in the back of his helmet by a 92 mph fastball in a game against the then-Florida Marlins.
He suffered a concussion and was later diagnosed with vertigo. He never made it back to the big leagues despite playing more than 300 games in the minors after the beaning.
His tale began to draw national attention this year, thanks to campaign by filmmaker Matt Liston, and the Marlins then offered him the one-day contract.
Greenberg's salary for the day will be donated to help the study, treatment and prevention of the effects of brain trauma in athletes.