Pachuca takes a 1-0 lead over Monterrey into Sunday's Liga MX Clausura final second leg, after Argentine Franco Jara marked the difference between the sides in Thursday's first leg.
Los Tuzos were worthy winners against regular season first-place finishers Monterrey, who took until the 86th minute to even record a shot on goal in Estadio Hidalgo.
Jara lost marker Jose Maria Basanta and headed in 20-year-old Pachuca captain Erick Gutierrez's cross in the 61st minute for the only goal of the game.
Four minutes earlier, Pachuca had complained ferociously to the referee after a Jonathan Urretaviscaya shot was blocked on the line by Monterrey midfielder Efrain Juarez. TV replays showed the ball did strike the upper part of Juarez's arms and a penalty should've been called.
With rain falling heavily and the ball difficult to control on a slippery surface, both teams found it challenging to play the ball around as they are used to, making for an open first half.
Colombian Edwin Cardona had the best early chance for Monterrey when he found himself free inside the penalty area from a Dorlan Pabon cross, but his header sailed wide of the goal.
It was a rare glimpse at goal for Monterrey, as Pachuca began to assert its authority. U.S. International Omar Gonzalez -- left out of his country's Copa America squad -- saw his header saved in the 25th minute and Jonathan Orozco was again called into action just after to tip over a Urretaviscaya shot from the right-hand side of the penalty area.
The teams went into half-time at 0-0, but Colombian international center-back Oscar Murillo didn't return for Pachuca after the break with what looked like a groin strain, potentially putting his Copa America participation in doubt.
Monterrey played without one of its best players of the season in Carlos Sanchez, who joined up with the Uruguay squad to prepare for the Copa America earlier this week under controversial circumstances. Los Rayados seemed to miss his drive and penetration in the attacking third.
Pachuca will try to win its sixth title this weekend, with Monterrey currently one behind them on four.
The second leg will take place at Estadio BBVA Bancomer on Sunday (9:30 p.m. ET), with the away goal rule not in place.