Leeds United came away from the Stadium of Light with a much-deserved 2-0 win in a game which showed the importance of a strong squad in the Championship.
It looked to be a game too far for Sunderland's first choice side, picked again for the fourth league match in a row.
They looked a touch off the pace, easily beaten by an impressive Leeds side. Goals from Samuel Saiz and Stuart Dallas settled a disappointing contest, with the Black Cats only really threatening when Lewis Grabban had an effort cleared off the line with the scores still level.
Saiz, in particular, suggested that the visitors may not struggle as much as expected when Chris Wood leaves for Burnley.
The opening goal was a cruel reality check for Sunderland, who had started in the same positive manner that had enthused supporters on the opening night of the season.
Grabban twice went close, first just unable to reach a fine cross from Brendan Galloway, played in down the left by Didier Ndong. Just minutes before Leeds went ahead, the Sunderland striker hit the woodwork after starting a fine counter with a lofted pass for James Vaughan. Vaughan returned the ball to his onrushing team-mate, who chipped the keeper but saw Liam Cooper nod his effort onto the crossbar.
The visitors were missing star striker Wood, on the verge of joining Burnley, but highly-rated summer signing Saiz showed his worth with a fine finish in the 21st minute.
Winger Ezgjan Alioski won the ball from a throw-in and cut the ball back for the Spaniard, who fired across goal into the bottom corner. Alioski was the biggest threat throughout the half, and Sunderland's stark weakness down the left flank eroded the confidence and saw them fall away badly.
Alioski tormented Brendan Galloway, first crossing for Pablo Hernandez, who headed over, then firing wide himself from inside the box. Leeds continued to control the game in the second half, forcing Simon Grayson to introduce Adam Matthews and Wahbi Khazri before ten minutes had passed.
Galloway, who made way for Matthews, looked to be struggling with an injury.
The Black Cats had improved but were struggling to create opportunities, with Felix Wiedwald untested in the Leeds goal. The end product simply wasn't there, crosses too flat and too many passes misplaced.
The visitors may have scored with their first meaningful attack of the second half, but the way they picked a tired Sunderland off on the break was still no great surprise.
Saiz received the ball on the right, perhaps a fraction offside, before crossing to the back post. The run of substitute Dallas was not tracked and his header was a simple one. Sunderland did threaten in the closing stages,
Billy Jones missing a glorious opportunity to score at the back post, but on the whole this served as a reality check after such a promising start to the season.