Victor Lopez scored the only goal 10 minutes from time after the match at the Marcelo Bielsa stadium was interrupted for 23 minutes in the second half due to a lighting failure.
Newell's enjoyed more possession but missed the firepower up front of top scorer Maxi Rodriguez, who was away on international duty with Argentina.
The introduction of substitute David Trezeguet, on loan from River, was the catalyst for the goal scored seconds after the former France striker came on.
A free kick from the left was headed upwards by Newell's midfielder Pablo Perez and central defender Lopez nipped in to head home from one metre past goalkeeper Marcelo Barovero.
River claimed they showed have been awarded a penalty in added time when striker Federico Andrada went down in the box - and TV replays appeared to show he had his ankle clipped by Gabriel Heinze - but the referee waved play on.
Newell's remain three points ahead of Arsenal with San Lorenzo and Boca Juniors three points further back.
Boca's push to keep pace with the leaders suffered a setback when they were held 1-1 by Rosario Central at La Bombonera.
Boca went ahead in the 13th minute when striker Emmanuel Gigliotti scored his fifth goal in five matches with a low shot from outside the box.
Central's deserved equaliser came in the final minute from veteran former Uruguay striker Sebastian Abreu and was the first Boca have conceded in five matches.
"We have to admit they played better than us, they controlled the match but we were 1-0 up and had three chances to settle the match," Boca coach Carlos Bianchi told reporters.
Bianchi said that being without midfielder Fernando Gago, who is nursing an injury, was no excuse for the dropped points.
RACING LOSE AGAIN
"We had a bad day just when he wasn't there but we must try to play well without him, or Roman (Riquelme)," Bianchi said.
Racing Club's record worst start to a season was extended to 11 games without a victory after a 1-0 defeat at Estudiantes left them at the foot of the standings with just two points.
Estudiantes ended a run of six matches without a victory thanks to an own goal by Racing defender Jose Luis Gomez less than a minute after halftime.
Racing were under caretaker Nacho Gonzalez, in charge for one match before Reinaldo Merlo's return for a third spell in charge from Monday.
Merlo, the team's third coach this season after the departure of Luis Zubeldia and Carlos Ischia, steered Racing to the last of their seven league titles in 2001.
Arsenal came from a goal down to beat nine-man San Lorenzo 2-1 at home on Saturday with two late goals from former Saints striker Julio Furch.
San Lorenzo's midfielder Leandro Navarro was sent off in the 48th minute and defender Santiago Gentiletti in the 64th, both for second bookings.
- Sports & Recreation
- Marcelo Bielsa
- Maxi Rodriguez
- David Trezeguet