Former Bayern Munich forward Guerrero of Peru nodded into the corner of the Egyptian side's net on 28 minutes from a gorgeous piece of skill by Douglas, who delivered the ball with the outside of his left foot almost like he was in a training session.
The South American champions - bidding for a second Club World Cup success after winning the trophy in 2000 - were dominant in the first period, but strangely fell out of matters in the second half as Cairo's Al Ahly huffed and puffed, but ultimately lacked the quality needed to find an equaliser despite never lacking willingness.
Ramy Rabia wasted their best chance of the match when he headed wide of goal in space from Walid Soliman's free-kick on 11 minutes.
They would have plenty of possession after half-time, but lacked the intricacy required to unfasten the centre of the Corinthians defence which left them largely restricted to speculative efforts from distance.
Ahmed Fathi also spurned an obvious chance when he hurried a shot on 65 minutes with goalkeeper Cassio racing from his line for no apparent reason, but the shot would end up in the side netting despite Cassio's barminess.
Chelsea and their new coach Rafael Benitez will not be overly worried about the goings on from this contest, but have work of their own to do to reach the final when they face Mexican club Monterrey on Thursday.
Chelsea midfielder Oscar has been busy making his companions aware of what to expect from Corinthians should they reach the final.
Oscar said: "In Brazil, this has always been seen as the one opportunity for our clubs to play against one of the big European teams. That opportunity doesn't come around very often, and this was always seen as the chance for them to test themselves against a big opponent from Europe, and to go and show their football to the world."
Corinthians were watched by an estimated 20,000 vociferous travelling fans. They qualified for the tournament via their success in lifting the Copa Libertadores, and are bidding to become the 10th Brazilian winners of the Club World Cup since it was first played in 1960.
Teams from Argentina, Brazil and Italy have each won the tournament nine times.