Christian Benitez scored an 89th-minute winner as Santos Laguna edged 2-1 ahead after the first leg of their Mexican Apertura semi-final against Club America.
Carlos Quintero had fired visitors Santos Laguna in front after nine minutes, pouncing on a deflected shot and arrowing a precise shot into the bottom left corner from 15 yards.
Club America then got lucky with their equaliser ten minutes later when Angel Reyna looked a couple of yards offside as he received a pass from midfield before slotting home a composed finish.
The goal looked set to earn them a draw until Benitez had his late say, cracking a 25-yard low shot through a thicket of Club America defenders and into the left corner, to put Santos Laguna in a strong position to go through to the final.
In the other semi-final, Unam Pumas and Monterrey played out a goalless draw. The second leg matches take place on Sunday.