Messi tees up victory as Barcelona cling on to fading title hopes
Messi teed up Arturo Vidal's winner at Jose Zorilla to become the first player to provide 20 assists in Spain's top flight since Xavi Hernandez in 2009.
Victory for Barca means Real Madrid cannot clinch the title on Monday by beating Granada but only an incredible collapse will prevent them putting their name on the trophy later in the week.
If they beat Granada, Madrid will win La Liga by prevailing at home to Villarreal on Thursday, regardless of Barcelona's results.
Overall, Madrid need only five points from their remaining three matches to secure their third La Liga title in eight years.
Yet even as Barca's turbulent domestic campaign draws to a close, Messi continues to shine.
His delightful pass for Vidal in the 15th minute, a scoop between two Valladolid defenders, triggered more remarkable milestones.
As well as matching the best La Liga total for assists by one player in 11 years, which he will surely break with two games still left to play, Messi extends his own career-best number of assists in a single league season.
He also becomes only the second player in the 21st century from the five major European leagues to hit 20 goals and 20 assists in the same season, after Thierry Henry scored 24 goals and laid on 20 assists in 2002-03 for Arsenal.
Perhaps most impressively, and worryingly for Barca, Messi has either scored or assisted 24 of their last 31 goals in La Liga.
The 33-year-old has halted negotiations to extend his current contract, which expires in 2021, perhaps to express his frustration with the running of the club.
As Barcelona's title challenge has wilted, Quique Setien's position as coach has come under scrutiny too, despite him only taking over in January.
But Setien was bold against Valladolid, deploying a 3-5-2 formation, with Nelson Semedo and Jordi Alba as wing-backs and the versatile Sergi Roberto dropping into the back three.
Luis Suarez started on the bench but replaced Antoine Griezmann at half-time, the Frenchman enduring one of his quieter displays and miskicking an excellent chance from close range.
After playing Messi behind Suarez and Griezmann against Villarreal on Sunday, Setien appears to be experimenting, perhaps with an eye on the Champions League's resumption next month.
Barcelona have a second leg at home to Napoli to negotiate after a 1-1 draw in Italy in the first leg, with a potential quarter-final to follow against the formidable Bayern Munich, assuming they finish the job against Chelsea.
The system worked well in the first half but Barca's problem of converting possession into chances remained and Valladolid grew in confidence late on.
Griezmann snatched at a finish inside the box before Vidal showed him how it was done.
Messi recovered the ball on the edge of the area and after receiving it back from Semedo, he scooped it through to Vidal, who gathered and unleashed all in one movement to find the far corner.
Valladolid's Kike Perez raced clear of Gerard Pique but lost his footing at the crucial moment before Messi weaved through but drove wide.
Marc-Andre Ter Stegen had to make two saves in the second half as Valladolid pushed, the first from an Enes Unal header and the second in injury-time, after Sandro Ramirez was allowed space at the back post. A weary Barca held on.