Leaders River Plate were held to a 1-1 draw by Boca Juniors in atrocious conditions on a waterlogged Monumental pitch in Argentina's "Superclasico".
Playing in a downpour for most of the match, both goals were scored by central defenders, Boca taking the lead through Lisandro Magallan at a 22nd minute free kick.
River, whose Uruguay striker Rodrigo Mora blasted a first half penalty over the bar, equalised 12 minutes from the end through substitute German Pezzella to the joy and relief of the home support in a 60,000 crowd.
The defender, who had come on two minutes earlier, tapped the ball home from the rebound after goalkeeper Agustin Orion had parried his header.
Boca's Argentina World Cup midfielder Fernando Gago was sent off in the 41st minute for handball preventing a River chance from finding the net, giving away the penalty and leaving his team a man short for more than half the match.
"We played a great match with a man less. In my opinion we lost two points, we could have won," Magallan told reporters.
Mora's penalty miss was probably poetic justice, though, since Gago claimed he did not touch the ball with his hand, only his head, and video replays appeared to bear him out.
Earlier, River right back Gabriel Mercado had hit the post with a header from a corner.
River centre back Ramiro Funes Mori was shown the red card in the 84th minute for a boots first tackle on Boca midfielder Marcelo Meli.
'IMPOSSIBLE TO PLAY'
Boca appeared to adapt better to the poor conditions than River, who have impressed on dry pitches with their fine attacking football, scoring 22 goals in their 10 matches.
"It was impossible to play, it was a shame for the show," said River coach Marcelo Gallardo, whose unbeaten side have drawn their last three matches after winning the previous six but stay top of the standings with 22 points from 10 matches.
"When you can't play you have to be intelligent," he added about sending on Pezzella for creative midfielder Leonardo Pisculichi, whose style was cramped by the conditions, as River sought to bombard Boca's box with crosses.
The result lifted Boca into the top half of the standings with 14 points, 11 coming in six matches since coach Rodolfo Arruabarrena took over an ailing side from Carlos Bianchi.
Second-placed Independiente can climb to within a point of River if they win at Godoy Cruz on Monday when Lanus, a point further back, visit Rosario Central.
Racing Club, with 16 points, were due to host Atletico Rafaela later on Sunday (0030 GMT), weather permitting.
