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The league leaders started impressively, picking up the ball in good areas and delivering dangerous crosses into the box. Karl Darlow and the Forest defence dealt with the pressure well though.
Cardiff fashioned their first good chance 15 minutes in, when Helguson collected his own rebound to fire inches wide from the edge of the box.
He wasn't to be denied for long though, meeting Craig Bellamy's free kick to steer the ball home from six yards out.
Forest thought they were level five minutes later when Lewis McGugan curled the ball in from just outside the box. However, the referee had already give a foul against the midfielder for pushing Aron Gunnarsson.
With minutes to go until the break, the away side were reduced to 10 men. Andre Marriner judged that Henderson had thrown an elbow at Helguson, and produced a red card.
Cardiff fashioned another chance but Helguson missed from close range to ensure it is only 1-0 at half time.