Barcelona saw their unbeaten start in the Primera Division brought to an end as Athletic Bilbao claimed a deserved 1-0 victory at San Mames Barria on Sunday night.
Iker Muniain steered home the only goal of the contest 20 minutes from full-time as Barca slipped to a second loss in five days following their disjointed defeat to Ajax in the Champions League on Tuesday.
While the loss in Europe can be absorbed, defeat in La Liga is much more damaging with Gerardo Martino's men now level with Atletico Madrid at the summit on 40 points, three clear of a Real Madrid side who appear to finally be hitting their straps.
Brazilian striker Jonas hit an eight-minute hat-trick as Valencia cruised past 10-man Osasuna 3-0 at the Mestalla.
Jonas scored after 45, 48 and 53 minutes as the home side finally ground out a win against opponents who had seen captain Francisco Punal sent off in the 16th minute.
Alberto Bueno's injury-time equaliser meant Rayo Vallecano shared the spoils with Real Betis in an exciting bottom-of-the-table clash at the Benito Villamarin stadium.
Betis looked to be heading for their first league win in two months when Joan Verdu scored with 11 minutes remaining, but in the 91st minute Bueno struck for the second time in the game to earn Rayo their first draw of the season.
Antonio Amaya had given the hosts a first-half lead which was cancelled out by Bueno early in the second period.
Betis entered the game at the bottom of the league, with Rayo just one place above them, but Paco Jemez's side move one point clear of the relegation zone thanks to the draw, while the Andalusians stay rooted to the bottom.
In the day's other game, Carlos Bacca scored the opener and then teed up Kevin Gameiro's late winner as Sevilla stole a 2-1 victory from their Andalusian rivals Granada.
Bacca fired Sevilla into a 23rd-minute lead but when Yacine Brahimi equalised from the penalty spot on the hour it was Granada that looked the most likely winners.
The home side pressed for a winner only to be caught out three minutes from the end by substitute Gameiro as Sevilla made it a hat-trick of wins in the league.