The opening minutes failed to bring any clear openings at either end, with the visitors having to make do without star striker Fernando Torres, with a knee injury ruining his return to his former club.
However, that changed nine minutes in as Atletico made the breakthrough, after the Reds defence failed to clear their lines following Jose Manuel Jurado's cross.
Forlan made a mess of his initial header, but prodded home from close range at the second time of asking when the ball fell at his own feet.
After 18 minutes Yossi Benayoun had the ball in the net, but an offside flag spoiled the midfielder's celebrations and soon after Steven Gerrard fired wide after running on to a pass from Lucas.
As the half wore on the play became disjointed and there were no further chances for either side.
After the restart play continued to be scrappy, although Reds skipper Gerrard started to pose problems further forward.
Atletico then came close to adding a second when Tomas Ujfalusi's cross from the right found Simao, who got his shot on target despite slipping over and Jose Reina was forced into a world-class one-handed tip-over.
Liverpool then brought on Ryan Babel in place of the ineffectual David N'Gog, but the Dutchman struggled to make any impact.
Reina came to his side's rescue again 15 minutes from time when he got down well to block Ujfalusi's left-foot drive, with Jamie Carragher lashing the loose ball clear.
That proved to be the final opening of a cagey encounter which left the tie delicately poised.