Messi opened the scoring with an unstoppable free kick then combined with devastating effect with Luis Suarez to carve Betis open and score a second goal right before halftime.
European champions Real Madrid picked up a record fifth trophy of the year to cap a fine 2017 at the Club World Cup last weekend, but trail Barca by a mammoth 11 points at the top of La Liga albeit with a game in hand.
The five-times world player of the year agreed the new contract with Barcelona in July but the Catalan club did not release the usual photo of the player putting pen to paper, fuelling speculation he could depart in June 2018.
Barcelona's 100 percent start to the Liga season will be severely tested at Atletico Madrid in an atmosphere certain to be intensified by political fervour.
Neymar signed a new five-year deal with Barca in 2016, but the Catalans were powerless to stop his move to PSG just a year later.
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