Swansea City's Wayne Routledge struck late to hand the visitors a 1-0 victory and three points at Elland Road as Leeds United lost their unbeaten league record.
Marcelo Bielsa made one change from United's last league outing as captain Liam Cooper returned in defence for Gaetano Berardi.
It was United who started the better of the two sides as Gjanni Alioski twice went close from inside the box with his second attempt forcing Freddie Woodman into his first action of the afternoon.
Patrick Bamford also flashed a header wide of the mark as the half continue following good work from Stuart Dallas on the right.
Former Whites loanee Matt Grimes went close for the visitors with the free-kick as the Swans were restricted in their attempts at goal.
On the stroke of half-time Bielsa's men almost broke the deadline as the returning Cooper with a powerful header we crashed against the crossbar.
The central defender fired the rebound over as the scores remained level at the interval.
Bamford and Hernandez again went close after the break as Bielsa responded by replacing the former with Arsenal loanee Eddie Nketiah.
Jack Harrison then saw another opportunity spurned after good work from Hernandez bundled the ball to the winger.
Helder Costa was thrust into the action and the substitute almost made an immediate impact as he tee'd up Hernandez on the edge of the box with the Spaniard's strike whistling past the post.
Costa again was the architect as Nketiah this time lost his marker at the back post as his header fell agonisingly just past the post.
As time expired disaster struck for the Whites as Swansea won themselves a rare corner.
Steve Cooper's men battled as the ball bounced around the box before substitute Wayne Routledge gathered himself to strike a low shot into the bottom corner past Kiko Casilla.
United were stunned and failed to muster a response as it was the visitors who left West Yorkshire with all three points.
Leeds United: Casilla, Alioski, White, Cooper (c), Dallas, Phillips, Klich, Harrison, Forshaw, Hernandez, Bamford. Subs v Swansea City: Meslier, Berardi, Douglas, Shackleton, McCalmont, Costa, Nketiah.
Swansea City: Woodman; Roberts, van der Hoorn, Rodon, Bidwell; Fulton, Grimes (c); Ayew, Dhanda, Celina; Baston. Subs: Nordfeldt, Naughton, Wilmot, Byers, Garrick, Routledge, Surridge.