The Swans went down 1-0 at Stoke City as Joe Allen's controversial second-half goal inflicted a first away defeat on Graham Potter's side.
Former Swans Allen tucked home the decisive goal in the 57th minute, but only when Tom Ince's shot deflected off Benik Afobe - who was offside - and into the path of the Welsh international.
It was harsh on the Swans, who had battled bravely at the bet365 Stadium.
Defeat leaves Potter's side on 12 points from eight games.
The Swans made seven changes to the side which began against Nottingham Forest.
Cameron Carter-Vickers made his first start in central defence while Martin Olsson returned at left-back.
Kyle Naughton played in midfield while experienced wingers Nathan Dyer and Wayne Routledge made their first appearances of the season.
George Byers, 22, made his Swans league debut while Daniel James was recalled in the final change.
The visitors had started brightly in Staffordshire, with Routledge's header deflected for a corner before Byers failed to connect with Dyer's cross along the face of goal.
At the other end, Afobe was denied by Erwin Mulder after he had latched onto Ryan Shawcross' long ball before the striker fired wide from a great position.
Stoke wasted another glorious opportunity before the break, with Ince - just 12 yards out - curling his low effort an inch wide of the post.
Despite the Swans' advances after the break, it was Stoke who broke the deadlock before the hour via Allen.
The Swans pushed for an equaliser, with Oli McBurnie, Yan Dhanda and Joel Asoro thrown on.
It was McBurnie who teed up James for a pop at goal, but the youngster's shot was blocked.
And so Stoke held on for a second league of the season - inflicting a second league defeat on the Swans.
Stoke City: Jack Butland, Cuco Martina, Ryan Shawcross (capt), Bruno Martins Indi, Erik Pieters, Ryan Woods, Joe Allen, Peter Etebo, Tom Ince, Mame Biram Diouf (Saido Berahino, 75), Benik Afobe.
Subs: Adam Federici, Tom Edwards, Ashley Williams, Darren Fletcher, Peter Crouch, Bojan.
Swansea City: Erwin Mulder, Connor Roberts, Cameron Carter-Vickers, Mike van der Hoorn (capt), Martin Olsson, Kyle Naughton, Matt Grimes, Nathan Dyer (Yan Dhanda, 64), Daniel James (Joel Asoro, 72), Wayne Routledge, George Byers (Oli McBurnie, 64).
Subs: Kristoffer Nordfeldt, Joe Rodon, Declan John, Barrie McKay.