Cristal won the Clausura phase play-off over Alianza Lima 1-0 at the Estadio de la Universidad Nacional San Agustin in Arequipa - with the contest played on neutral territory.
Daniel Ahmed's Cristal will enter the two-legged championship decider against Juan Aurich, starting Monday.
The decisive moment on Thursday came in the 58th minute, when Sergio Blanco was on hand to lash home a chested-down ball from Maximiliano Nunez.
Blanco had earlier spawned a brilliant chance to open the scoring, when he was put through by Irven Avila in first-half injury time - but he pulled his left-foot strike well wide of the target.
However, the Argentine made no mistake prior to the hour mark, as he gave Cristal the chance to make it two league titles in three years.
Also on Friday, Los Caimanes were relegated to the Segunda Division after they were humbled 1-0 by Sport Huancayo in extra-time at the Estadio Julio Lores Colan in Huaral.
Jankarlo Chirinos' goal in injury-time of the second half of the additional 30 minutes saw Huancayo survive in the top flight.
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