Alianza Lima conceded 12 minutes from time as fourth-placed Union Comercio emerged 1-0 victors in the penultimate Peruvian Primera Division fixture at the Estadio del IPD de Moyobamba on Wednesday.
After a goalless first half, an unmarked Jesus Chavez was the hero for Union Comercio, side-footing past goalkeeper George Forsyth to seal the points 12 minutes from time.
The defeat left second-placed Alianza Lima two points adrift of leaders Sporting Cristal after 14 rounds.
Sporting Cristal travel to Los Caimanes for their final Clausura outing on Sunday, while Melgar host Alianza Lima.
Lowly Universidad San Martin were also triumphant on Wednesday, coming from behind to claim a rare 2-1 win at home to Inti Gas.
Universidad San Martin - second from bottom in the table - trailed 1-0 at half-time on the back of Oscar Guerra's 31st-minute opener.
But the home side hit back, with Luis Perea equalising six minutes into the second half before Santiago Silva converted an 89th-minute penalty for Universidad San Martin's third win of the season.
Meanwhile, Leon de Huanuco extended their winning streak to four matches thanks to a 1-0 victory over Cesar Vallejo, who lost for the first time in three games.