An own goal from Carlos Izquierdoz ensured the first leg of the Copa Libertadores final between Boca Juniors and River Plate ended in a 2-2 draw.
The home side twice surrendered the lead against their fierce rivals in an enthralling encounter at the boisterous La Bombonera in Buenos Aires.
The much-anticipated clash, the first between Argentina's biggest clubs in the final of South America's top club competition, was delayed until Sunday due to heavy rain in the capital.
Boca went ahead through Ramon Abila only to see Lucas Pratto level practically from kick-off, and their advantage was again snatched from their grasp when Izquierdoz's wayward header cancelled out Dario Benedetto's goal.
It leaves the tie tantalisingly poised ahead of the second leg in 13 days' time.
It was River who threatened first, Agustin Rossi keeping out Gonzalo Martinez's free-kick before Lucas Martinez Quarta made a mess of a free header from the resulting corner.
Rossi them denied Santos Borre with a fine one-handed stop, and Boca's difficult start got worse when Cristian Pavon was forced off after 27 minutes with what looked like a hamstring injury.
The Bombonera was rocking 34 minutes in, though. Abila saw a shot saved by Franco Armani, but fired the rebound through the grasp of the keeper and in.
The lead lasted barely 60 seconds, with Pratto racing on to Martinez's pass and firing across goal and into the bottom-right corner.
Another chance for Martinez came and went before Boca snatched the lead again seconds before half-time, Benedetto glancing Sebastian Villa's deep free-kick high into the net.
A similar set-piece routine almost created a third for the home side, only for River to equalise again in similar fashion, when Izquierdoz headed in past the helpless Rossi from six yards out.
Boca introduced Carlos Tevez from the bench and the veteran striker almost provided the winner, first lashing a shot just wide of the left-hand post before teeing up Benedetto to shoot straight at Armani with the goal at his mercy.
What does it mean? River hold slender advantage
Away goals do not count in this final, but River will be happy to head into the home leg knowing a win by any scoreline will be enough to see them clinch the trophy.
The manner in which they responded to falling behind twice will have emboldened coach Marcelo Gallardo, even if he was banned from being present at the match.
Boca will be frustrated not to have held on to the lead on either occasion, and the injury to Pavon will give them more cause for concern ahead of the second leg.
Rossi bails out Boca when it matters most
Rossi only made four saves in total, but all could prove hugely important after 180 minutes of this tie have been played. He could do little about the goals, either.
3 - Agustín #Rossi made three saves in the FH against #River Plate; the most for a #Boca Juniors' goalkeeper in a Copa #Libertadores 2018 first half. Vital. https://t.co/ELUcOGwYCJ — OptaJavier (@OptaJavier) November 11, 2018
Martinez Quarta a mess
Having missed a clear chance with a header early on, Martinez Quarta was caught out of position as Abila broke the deadlock. He was taken off after 58 minutes.
All eyes are on November 24, when River will host the second leg at the end of the international break.