Barcelona let off Real Madrid again and went four League games without a win as Valencia held them to a 1-1 draw on Sunday.
Valencia took a second-minute lead through Ezequiel Garay, but more Lionel Messi magic bailed out Barca, although the Catalans ultimately slumped to their worst run of La Liga form for over two years.
The form of Philippe Coutinho and Arthur kept Ousmane Dembele out of Barca's starting XI, but Malcom was not even a substitute.
Valencia, meanwhile, won their first game of the season last weekend against Real Sociedad, before holding Manchester United to a goalless draw in the Champions League.
Los Che were practically full-strength as only Santi Mina was missing with a knee injury, although they were well stocked in attack with Goncalo Guedes, Kevin Gameiro and Michy Batshuayi.
Barca had not lost at Mestalla since 2008 but had gone three Leagues games without a victory and went down 2-1 to Valencia in April 2016, as part of a four-match winless streak.
Valencia applied pressure from the get-go to force an early corner, which took nicks off Thomas Vermaelen and then Gerard Pique, gifting Ezequiel Garay the game’s first goal from a yard out.
The hosts could’ve been out of sight in the couple of minutes that followed, but Michy Batshuayi's angled drive was tipped around the post by Marc-Andre ter Stegen and Geoffrey Kondogbia’s curler flew a fraction wide of the German’s goal.
Marcelino’s side seemed to have spared when Jose Gaya wasn't deemed to have tripped Luis Suarez in the box, but Messi nonetheless nutmegged Kondogbia and played a one-two with Suarez, before picking his post in Neto’s bottom-right corner.
Denis Cheryshev replaced Goncalo Guedes earlier than planned, the Portuguese having picked up a thigh strain, and his shot from a Kevin Gameiro cutback was blocked. It turned into an opening for Jose Gaya, whose strike flashed across Ter Stegen.
The Blaugrana dominated possession throughout the second half, almost trebling Los Che’s number of passes, but Coutinho was lucky to avoid a red card for two slaps to Dani Parejo’s face.
Ernesto Valverde threw on Dembele and Rafinha late on, but all his team could muster was a drilled Messi effort from a tight angle, which arrowed the wrong side of Neto's near post.
Barca’s failure to beat Valencia allowed Sevilla to take top spot in La Liga as the Andalusians chained together a fourth straight win.
Valencia 1-1 Barcelona
Garay 2 (V) Messi 23 (B)
Valencia: Neto; Piccini, Garay, Gabriel, Gaya; Soler, Parejo, Kondogbia (Coquelin 79), Guedes (Cheryshev 12); Batshuayi, Gameiro (Rodrigo 67)
Barcelona: Ter Stegen; Semedo, Pique, Vermaelen, Alba; Rakitic, Busquets, Arthur (Rafinha 88); Messi, Suarez, Coutinho (Dembele 84)
Referee: Gonzalez Gonzalez