Monaco had never beaten the Spaniard in two prior meetings and it looked like he would fall again, but used a dominating final two sets to take the win and will now try to reach his third finals at this event.
He also reached the title contest of this clay-court event in 2010 at Santiago, and 2008.
Chela, the second seed, outlasted Brazil's Joao Souza 5-7, 7-6 (8-6), 6-1 in a match that lasted three hours. The Argentinean staved off 10-of-13 break chances by Souza and converted on 4-of-8 of his own, including three in the third set, en route to the semis.
In other quarterfinal action, seventh-seeded Carlos Berlocq grounded qualifier and fellow Argentine Federico Delbonis 6-3, 6-4, and Frenchman Jeremy Chardy downed Portugal's Frederico Gil 6-2, 7-6 (7-5).
Monaco will face off against Chardy in the semis, while Chela will do battle with Berlocq.
The 2012 Vina del Mar champion will collect $71,900.