Manchester United were denied a precious first-leg lead by Porto as Mariano grabbed a deserved last-minute equaliser in a 2-2 draw.
After trailing to Cristian Rodriguez's fourth-minute opener, United had profited from a large slice of luck when Wayne Rooney netted the equaliser.
And when Rooney set up Carlos Tevez to put the Red Devils in front five minutes from time it seemed Sir Alex Ferguson's men would take the narrowest of leads to Portugal next week, only for an unmarked Mariano to deny them at the death.
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