A second-half surge from River Plate saw the Argentine club take a significant step towards the Copa Libertadores final on Tuesday.
River - aiming for the third Libertadores title in their history - were held scoreless in the first half of their semi-final first leg against Guarani in Buenos Aires, but Gabriel Mercado and Rodrigo Mora struck after the interval to secure a 2-0 victory.
Mercado pounced on Lucas Alario's knock-down on the hour mark, while Mora produced a sublime chip 13 minutes later to wrap up the win.
Guarani, who have never played in a Libertadores decider, will host the second leg in Paraguay next week.
Marcelo Gallardo's River entered Tuesday's home leg looking for their first final berth since 1996 when they lifted South America's premier club trophy.
But they had to wait until the 60th minute to break open Guarani's defence, with striker Alario getting on the end of a corner and nodding the ball towards Mercado and the full-back converted from less than 10 yards.
In the 73rd minute, Mora burst clear of the visitors' defence and surged into the box before chipping Alfredo Aguilar with a shot that bounced in off the underside of the bar.
The second Libertadores semi-final will begin on Wednesday between Brazil's Internacional and Tigres UANL of Mexico.