Substitute Diego Costa scored five minutes into his Atletico Madrid return in the Copa del Rey last-16 first-leg win at third-tier side 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 for Atletico.
"I'm really happy to feel like a footballer again and play once more with Atletico Madrid. It was very important for me," said Costa.
"I couldn't have hoped for a better start to my second spell with Atletico.
"I was given the opportunity to play again and I had the chance to score a goal, that makes me even happier."
Costa started the move that led to his goal, driving forward before moving the ball wide, allowing Juanfran to whip in a low cross that Costa clipped past Lleida keeper David Oliveros.
Costa appeared to have injured himself stretching to score the goal, but was able to carry on and finish the game.
"Costa's return has filled us with enthusiasm," said Atletico coach Diego Simeone. "He brings speed, strength and physicality, which is what we expected of him."
Brazil-born Costa joined Chelsea from Atletico for a reported £32m in 2014 and he scored 58 goals in 120 appearances for the Blues, including 20 in the Premier League last season as they won the title.
But he was told by text message he was not in Antonio Conte's plans, did not play for Chelsea this season, and spent much of August in his native Brazil.
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.