Rios made his debut in the World Wrestling Federation (WWF) on the November 3, 1997 episode of Raw is War as part of its newly relaunched Light Heavyweight division at the age of 18, wrestling under the Ring name Águila. This period included a match with Taka Michinoku at WrestleMania XIV for the Light Heavyweight Championship. He captured the WWF Light Heavyweight title from Gillberg on his 'debut' as Essa Ríos during February 13, 2000 edition of Sunday Night Heat. His reign was a short-lived one, as he would lose the title to Dean Malenko a few weeks later. During his time as Essa Ríos, he was managed by Lita who would often repeat Ríos's finishing moves on wrestlers he had just beaten after a match. He and Lita had a feud with Eddie Guerrero and Chyna which led to a European title match between Ríos and then-champion Guerrero at Backlash 2000, which Ríos lost. Ríos' partnership with Lita ended sometime later after an angry Ríos attacked Lita because he had lost a match. It was there that the Hardy Boyz came to Lita's rescue and feuded with him briefly, with Lita becoming their protégé. Ríos lapsed into obscurity but did have a brief feud with Kurt Angle that culminated in a main event match on Sunday Night Heat before once again only making occasional appearances. He was released in 2001.