Rayo Vallecano grabs 4th straight win
Alhassane Bangoura put Rayo ahead just after half-time and Medina Piti doubled the lead with a contentious penalty in the 65th minute as the visitors took control at San Mames.
Athletic halved the deficit through Mikel San Jose 13 minutes from time to set up a grandstand finish but, despite Rayo losing defender Alejandro Galvez to a red card late on, the Basque side could not get back on terms.
The victory moves Paco Jemez's men on to 31 points, the same as Malaga who face runaway league leaders Barcelona this weekend.
For Athletic, the loss is their third in succession and leaves them six points above the relegation zone.
Athletic came closest to a breakthrough in an evenly-fought first half with a header from Markel Susaeta that struck the post and an attempted lob from Aritz Aduriz that was parried Ruben Martinez.
However, it was Rayo who took the lead just three minutes into the second half through young forward Bangoura.
Roberto Trashorras picked out his team-mate's run with a lofted pass from inside the centre circle and the 20-year-old Guinea international took one touch to bring the ball down before smashing a low shot under Gorka Iraizoz.
Rayo almost doubled their lead four minutes later but Leo Baptistao was denied by Iraizoz.
Athletic coach Marcelo Bielsa decided to throw wantaway striker Fernando Llorente into the action in the 59th minute and the move almost paid immediate dividends with the home side twice coming close to an equaliser.
Aritz Aduriz almost found the back of the net with a deflected effort which Ruben did superbly to turn behind, and then from the resulting corner Llorente powered a header against the crossbar.
Athletic fans would have hoped that was a sign of better things to come, but three minutes later their side were 2-0 down after conceding a controversial penalty.
Alejandro Dominguez looked to be outside the area when he was brought down by Carlos Gurpegi, but the referee pointed to the spot and Piti slotted home from 12 yards.
Athletic refused to give up though and they pulled a goal back when San Jose headed home Markel Susaeta's cross.
The hosts charged forward in search of an equaliser and Llorente was unable to make the most of two good chances before Athletic's cause was aided by the 89th-minute dismissal of Galvez, who picked up a second yellow card for handball.
However, despite having the numerical advantage and the momentum behind them, Athletic could not find an equaliser as Rayo held on for the points.