Yamal opened the scoring in the sixth minute, lifting the ball over Stole Dimitrievski after Fermín López’s pass. Fermín added the second in the 22nd minute from Anthony Gordon’s cut-back. Raphinha tapped in Fermín’s cross at the start of the second half. Pedri made it 4-0 in the 79th minute and Yamal swept in his second four minutes later.
Barcelona have 12 points from four matches and 17 goals. Madrid lost at Betis on Friday and sit on nine. Atlético were beaten 3-0 at Athletic Club on Saturday.
Fermín finished with a goal and two assists. Yamal had a second early strike ruled out for offside. Rodri, starting after his move from Manchester City, headed against the bar. Eric García went off injured after a collision with Rodri. Gabriel Jesus, signed from Arsenal, made his debut from the bench and shot just wide.
Valencia were overrun. They managed little after the opening goal and finished with 26 per cent possession. The home crowd of more than 47,000 watched a one-sided title-defence statement.
Sunday completed a weekend in which Spain’s other heavyweights dropped points. Barcelona did not. Five goals at Mestalla keep them alone at the top. The next test is to sustain this scoring run; the first four matches have already set a fierce pace.