Guarani dreamed of a maiden Copa Libertadores final for about 15 minutes until Lucas Alario sent River Plate through.
River Plate advanced to their first Copa Libertadores since 1996 on Tuesday, with Lucas Alario's impressive chip securing a 3-1 aggregate victory over Guarani.
After going behind 1-0 in the semi-final second leg in Paraguay, Alario struck with 11 minutes remaining to end Guarani's resistance and send River into the continental decider.
The 1-1 draw at the Estadio Rogelio Lorenzo Livieres means River's dream of lifting their third Copa Libertadores remains alive, as they will face either Internacional of Brazil or Mexico's Tigres UANL in the final of CONMEBOL's premier continental club tournament.
After a scoreless first half, Guarani got themselves into the contest in the 62nd minute thanks to a goal from Fernando Fernandez.
A move down the right saw a cross whipped into the box and although Marcelo Pau's header deflected off the post and along the goal-line, Guarani retained possession and Federico Santander's follow-up cutback reached Fernandez at the back post, with the striker making no mistake.
Just six minutes later, Guarani saw a shot cleared off the line as they pushed for an equaliser, before River almost scored, with Alario's powerful header palmed over the bar by Alfredo Aguilar.
Alario made no mistake with his next significant chance, however, expertly lifting the ball over Aguilar on the edge of the box in the 79th minute from Tabare Viudez's pass.
River won the Copa Libertadores in 1986 and 1996, while they also lost finals in 1976 and 1966.