Diego Costa scored five minutes into his Atletico Madrid return after coming off the bench in the Copa del Rey first-leg win at Lleida Esportiu.
Chelsea agreed to sell Costa, 29, back to Atletico in September, although he was not cleared to play until January.
And he came off the bench for his second Atletico debut after 64 minutes, before scoring his side’s third goal.
Diego Godin, Fernando Torres and Antonie Griezmann were also on the scoresheet in the last-16 tie.
Brazil-born Costa joined Chelsea from Atletico for a reported £32m three years ago.
He scored 58 goals in 120 appearances for the Blues, including 20 in the Premier League last season as Chelsea won the title.
But the Spain international had not played for the Blues this season, and spent much of August in his native Brazil.
He was told by text message that he was not in Antonio Conte’s plans and expressed his desire to return to Atletico.
And while the deal was agreed in September, he could not be registered as a player for the La Liga outfit until January, when the club’s transfer window ban came to an end.
The second leg against third-tier Lleida Esportiu is on Tuesday, with Atletico hosting Getafe in the league on Saturday.