Ramon Abila opened the scoring in the 34th minute at La Bombonera Stadium with a powerful left-footed shot into the top left corner. But the lead didn't last long: Pratto equalised two minutes later, sending the ball into the bottom left corner.
Benedetto made it 2-1 to Boca with a header in first-half stoppage time before Boca defender Carlos Izquierdoz scored an own-goal in the 61st minute, attempting to defend a ball into the box.
River almost scored from the ensuing corner, but Lucas Martínez directed his header wide wastefully to leave La Bombonera breathing a collective sigh of relief.
River continued to enjoy the better of the early chances, and again threatened the Boca goal after 16 minutes when Rafael Borre's glancing header was brilliantly pushed around the post by Rossi for a corner.
However after soaking up the early River pressure, it was Boca who made the breakthrough on 34 minutes with their first shot of the game.
Ábila forced a desperate save from Armani with a fierce right-foot strike, but the River goalkeeper's block fell back into the path of the Boca forward who smashed an angled left-foot shot through a gap at the near post.
His opener had the massed yanks of blue-and-yellow clad fans bouncing in delirium but their joy was short-lived as River snatched an equaliser immediately.
Gonzalo Martínez threaded a perfectly weighted pass through to Pratto, and the Argentine international drilled a low finish across the diving Rossi which crept in off the far post.
River's star player played tribute to Boca's shotstopper Rossi.
"The Boca goalkeeper had two or three spectacular saves ... we were shooting at goal well but he was saving some very good shots," said Pratto, who was chosen as man of the match.
All eyes now turn to the second-leg showdown, due to take place in just two week's time. The two rivals will face off at the Monumental, River's home ground, on Saturday, November 24 (kick off 5pm local time).