Only an injury time goal got Banfield to the third round but another last-gasp strike may prove vital if El Taladro are to seal a place in the Copa Libertadores group stage after Darío Cvitanich salvaged a first leg draw against Nacional at the Estadio Florencio Sola.
Both sides had created chances in the first half with Banfield goalkeeper Iván Arboleda forced into one noticeably excellent stop.
However, it took just a few minutes of the second half for Nacional to make a breakthrough as Sebastián Fernández poked in at the back post following a corner.
Nicolás Bertolo headed an equaliser and Pablo Mouche perhaps should have given Banfield the lead but instead it was the Uruguayans to score next, Luis Espino converting from close range off the underside of the crossbar.
Banfield looked to be heading for defeat but in stoppage time, Nacional failed to deal with a corner and the ball eventually squirmed to Darío Cvitanich, who made no mistake in forcing the ball into the net.
Only a fine save by Esteban Conde denied Banfield a winner and so the two sides meet next week in Montevideo with everything still to play for. The Uruguayans will be favourites but El Taladro have already proved their character and will not lie down. A place in the Copa Libertadores group stages lies in wait.