Striker Cornelius, 23, hit back early in the second half, heading his own mis-hit shot to send the ball looping over Iker Casillas and into the net.
Copenhagen midfielder Jan Gregus was sent off for a second yellow card in the 66th minute but despite a flurry of chances in the final 24 minutes Porto could not make the advantage pay and the game ended in a draw.
(Reporting by Philip O'Connor in Stockholm; Editing by Clare Fallon)
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