Jose del Solar's Universitario conceded an 88th-minute goal as they lost 2-1 to UTC Cajamarca - previously winless - at the Estadio Heroes de San Ramon on Saturday.
Universitario were top of the standings heading into the round-six clash but find themselves in fifth spot on 11 points - two adrift of Inti Gas, who edged Union Comercio 1-0 on Friday.
Oscar Guerra was the hero for the visiting team, scoring six minutes before the half-time interval.
Universitario had enjoyed an unbeaten start to the season prior to Saturday's match - winning four of their opening five games.
But that record came to a halt after goals from Marcio Valverde and Manuel Corrales inspired UTC Cajamarca to their first win of the league campaign.
After a scoreless first half, the home team finally made the breakthrough courtesy of Valverde in the 63rd minute.
The Peruvian midfielder pounced on an attempted clearance, firing a volleyed effort from the edge of the box past goalkeeper Jose Carvallo, who was caught off his line.
UTC Cajamarca's joy was short-lived, however, as Raul Ruidiaz equalised six minutes later with a powerful header.
And just as both teams appeared set to share the points, Corrales sprung the offside trap to poke the ball beyond Carvallo two minutes from time.
In other results, Melgar are second in the table with 12 points - one behind Inti Gas - following a 1-1 draw at home to lowly Leon de Huanuco on Friday.
Juan Aurich and Cesar Vallejo are level on points with Universitario after both clubs drew.
Honours were even between 10-man Juan Aurich and Cienciano 1-1 on Sunday, while Cesar Vallejo played out a goalless draw at Alianza Lima on Saturday.
Real Garcilaso accounted for cellar-dwellers Los Caimanes 1-0.
Manuel Contreras levelled proceedings in the 63rd minute as second-bottom Sport Huancayo - trounced 6-0 by Juan Aurich last week - drew 1-1 with San Simon.
There were no goals scored in Universidad San Martin's trip to Sporting Cristal.
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