Barcelona breathed life into their title defence as Messi starred in their thrilling El Clasico win at 10-man Real Madrid.
Messi, Barca's all-time leading scorer, struck two second-half penalties as the Catalan giants came from behind twice to win a pulsating encounter at Santiago Bernabeu on Sunday.
Iniesta's early goal was cancelled out by a pair of Karim Benzema strikes, and although Messi levelled before half-time, a controversial Cristiano Ronaldo penalty put Real back in front.
Sergio Ramos was then sent off for bringing down Neymar, though, and Messi's first penalty restored parity.
And the Argentina star – who became the top scorer in Clasicos with his treble, overtaking Real Madrid legend Alfredo Di Stefano – held his nerve when presented with another penalty as his side notched their first league double over their rivals since the 2009-10 season.
And if Atletico and Real end the campaign on the same number of points, Diego Simeone's side will finish higher because of their win and draw garnered from the two derby meetings this term.
Atletico were made to work for their latest victory, but eventually their quality shone through against struggling Betis.
Costa had a goal ruled out for offside in the first half, a decision that appeared to be harsh, but the match swung in Atleti's favour when Braian Rodriguez was sent off for a second yellow card in the 56th minute.
Gabi capitalised with a stunning 25-yard strike just two minutes later, and Costa then showed his predatory instincts with a clever left-foot finish just after the hour as struggling Betis' slim chances of survival took another blow.
And Valencia beat Villarreal by the same scoreline – a result that helped them to eighth – thanks to Javi Fuego's double.
