Barcelona's title ambitions took a massive half brace to help Osasuna stun 2, at the Estadio El Sadar.
Lekic provided an early blow to the visitors when he scored in the fourth minute, while the advantage was doubled in the 21st when Lekic got on the other end of a low cross from Alvaro Cejudo to finish off the play at close range.
Barca found a way back into the match early in the second half with Isaac Cuenca's cross being converted from a few yards out by Alexis Sanchez, only for the hosts to restore their two-goal lead five minutes later with Raul Garcia knocking a shot off the near post and into the net.
Cristian Tello came off the bench to pull a goal back for Barca in the 73rd with a fine left-footed strike from inside the penalty area, but Osasuna continued to threaten as Francisco Punal struck the post.
The visitors poured on the pressure over the final 15 minutes, and after Sanchez had a goal ruled out for offsides, the Chile international was denied by a good save from Osasuna goalkeeper Andres Fernandez.
Barca's final chance to claim a point came deep into stoppage time when a header from Cesc Fabregas was saved by Fernandez from eight yards.
Pep Guardiola's men are now seven points back of leaders Real Madrid, who can move 10 points clear with a win on Sunday at home against Levante.
Real Betis 2, Athletic Bilbao 1
Seville, Spain - Real Betis moved three points above the drop zone on Saturday as Nelson's stoppage-time goal handed the home side a 2-1 win over Athletic Bilbao.
Ruben Castro fired Betis in front in the opening 10 minutes when he curled a shot inside the far post from 10 yards, only for Javi Martinez to cancel out the tally 12 minutes later with a headed goal from a corner kick for Athletic.
But Martinez received his second yellow card in the 63rd, and Nelson snatched all three points for his side with a well-struck volley from the edge of the penalty area to help Betis snap a run of three losses in four games.
Racing 0, Atletico Madrid 0
Santander, Spain - Atletico Madrid was held to a second successive 0-0 draw on Saturday, this time at Racing.
The visitors dominated play for long spells, especially in the second half, but were unlucky not to leave El Sardinero with three points.
Atletico was especially in control in the second half as Diego struck the post after exchanging passes with Arda Turan, while Radamel Falcao also wasted a great opportunity as he shot the ball against the post after collecting a rebound from Diego's shot.
A win could have temporarily lifted Atletico into the top four, but the club will have to settle for extending its unbeaten streak to six games.