Gremio looked set to lose for the first time in Group 6 on Wednesday after Maxi Rodriguez had given Newell's the lead with 11 minutes remaining.
But the Brazilian visitors responded in the first minute of second-half stoppage time when Rhodolfo nodded Hernan Barcos' cross into the net.
The draw saw Gremio (eight points) regain top spot after Atletico Nacional (seven) had claimed the lead on Tuesday with a 1-0 victory at Uruguay's Nacional (one).
After a goalless first half in Rosario, Argentina, Newell's gave the home fans something to cheer about in the 79th minute when Gremio failed to clear their defensive penalty area and Rodriguez pounced to slam the loose ball into the net.
Gremio's unbeaten run in the 2014 Copa Libertadores looked set to end until Barcos picked up the ball on the left wing, cut onto his right foot and his in-swinging cross glanced off Rhodolfo's forehead and into the bottom corner of the net.
The result left Newell's in third in Group 6 on five points.
In Group 7, Mexico's Leon thrashed Emelec 3-0 to leapfrog the Ecuadorians but had to wait until the final 10 minutes to wrap up the victory.
A powerful header from Matias Britos put Leon ahead in the 20th minute but the hosts did not double their lead until Jose Juan Vazquez's penalty with eight minutes remaining.
Just four minutes later and Carlos Pena made it 3-0 to give Leon their second win of the group stage, with the Liga MX outfit claiming top spot in their group with seven points.
Emelec (six points) sit second, while Bolivar (five) defeated Flamengo (four) 1-0 to rise above the Brazilian club.
Penarol claimed their first victory of the continental competition's group stage with a 2-1 triumph over Arsenal and defending champions Atletico Mineiro were held 1-1 at home by Nacional Asuncion.
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