La Liga leaders Atletico Madrid's title bid was derailed again as they were beaten 2-1 at Athletic Bilbao on Sunday by a late goal from Inigo Martinez, offering Barcelona the chance to go top of the table next week.
Martinez's bullet header from a corner in the 86th minute delivered a knockout blow to Atletico, who had pulled themselves level with a Stefan Savic header in 77th to cancel out Alex Berenguer's ninth minute opener.
Atletico lead the table on 73 points but third-placed Barca, on 71 after beating Villarreal 2-1 earlier on Sunday, can go top if they beat Granada on Thursday in their game in hand.
Atletico's defeat adds extra intrigue to a thrilling four-way title race, with three points separating them and fourth-placed Sevilla, who beat Granda 2-1, with five games left.
Champions Real Madrid are second on 71 after being held to a 0-0 draw at home to Real Betis on Saturday.
After a poor run of results let Real, Barca and Sevilla back into a title race, which had looked like Atletico's to lose, Diego Simeone's side had calmed their nerves with back-to-back wins over struggling Eibar and Huesca.
They faced an Athletic side without a win in any competition since March 7 and who lost two Copa del Rey finals this month, being hammered 4-0 by Barcelona in last week's showpiece.
But Athletic looked closer to the energetic and intelligent side seen in their first few weeks under coach Marcelino.
Berenguer put them in front with a diving header from close range, reacting quickly after Ander Capa's cross took a huge deflection off Atletico defender Felipe.
They missed several chances to increase their lead before halftime against a passive and disjointed Atletico.
Simeone sprung an attacking triple substitution after the break in search of an equaliser, throwing on top scorer Luis Suarez, record signing Joao Felix and winger Thomas Lemar.
The changes gave Atleti more urgency and led to them dominating most of the second half but they still lacked a cutting edge and struggled to contain Athletic's counters.
The hosts should have doubled their advantage when Jon Morcillo burst up the left flank and found striker Oihan Sancet, who blazed his shot over the bar.
Atletico got a lifeline when Athletic keeper Unai Simon misjudged Yannick Carrasco's corner and Savic rose to head home, but they could not build on the momentum and Martinez had the final say.