The former Bayern Munich striker netted for the Copa Libertadores champions to secure them a place in Sunday's showpiece fixture in Yokohama at the expense of El-Abry's side
The Brazilian side took a deserved 1-0 lead when the forward headed Douglas' inch-perfect cross past opposing goalkeeper Sherif Ekramy in the first half, and despite their African opponents improving after the break, an equalising goal never materialised.
Corinthians, making their first appearance in the tournament since winning the inaugural edition 12 years ago, had a reported 20,000 fans in the stadium, and it certainly sounded like an accurate figure as Timao's supporters spurred them on from kick-off.
It was the Copa Libertadores champions that fashioned the first shooting opportunity of the match; their attacking midfielder Douglas flashing a half volley just wide from a difficult position outside the Al-Ahly penalty area in the ninth minute.
That Douglas chance seemed to spark Al-Ahly into life as, after nearly going behind, they immediately went up the other end and threatened to break the deadlock themselves. Ramy Rabia was the player who came close, as his header from a fine Walid Soliman cross drifted off target.
Corinthians' first-half dominance finally paid off in the 30th minute when they went 1-0 up through Guerrero. The former Bayern Munich man headed Tite's side into the lead after connecting well with an audacious cross from the outside of Douglas' left boot.
The African champions were without their captain Hossam Ghaly after he limped off during their quarter-final victory over Sanfrecce Hiroshima on Saturday and despite the influential Mohamed Aboutrika taking his place in that game and scoring the winner, the 34-year-old was named on the bench this time around.
As Al-Ahly continued to struggle to create chances after the break, their coach Hossam El-Badry introduced Aboutrika in an attempt to revive his team and it had an instant impact on their attacking play.
Their best spell of the game soon followed, and it culminated with decent efforts from Rabia, Soliman and right-back Ahmed Fathy narrowly failing to find the back of the net.
Aboutrika's influence on the game grew in the second half, but his probing play failed to inspire an Al-Ahly equaliser, as the Brazilians held out for a narrow 1-0 victory.
Corinthians will now face the winner of the semi-final between Chelsea and Monterrey, with the final itself being held at the International Stadium in Yokohama on December 16.