The match looked headed for a goalless draw before the 32-year-old former Sunderland striker left his mark with eight minutes remaining on Tuesday, reports Xinhua news agency.
Gonzalo Martinez's long range shot was parried away by goalkeeper Esteban Andrada only for Scocco to rifle in the rebound with his left foot.
Despite playing just five of a possible 11 matches, Scocco has now netted seven goals to be the competition's second-top scorer behind the Strongest's Alejandro Chumacero.
The decisive second leg will be played at Ciudad de Lanus next Tuesday.
River Plate are aiming for a fourth Copa Libertadores title, following their triumphs in 1986, 1996 and 2015. Lanus have never won South America's most coveted club trophy.
The winner of the Copa Libertadores will earn the right to play in the FIFA Club World Cup in the United Arab Emirates in December.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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