Denmark went ahead after 57 seconds when Mathias Jorgensen scuffed the ball over the line off the foot of Croatia goalkeeper Danijel Subasic but Mario Mandzukic had the Croats back on level terms three minutes later.
If the 30 minutes of extra time fails to produce a winner, there will be a penalty shootout to decide which team goes on to face Russia in the quarter-finals in Sochi on Saturday.
(Writing by Nick Mulvenney in Kazan, editing by Christian Radnedge)
