The Tigers ended a 13-game winless streak as substitute Mason Burstow’s late goal saw off the Swans and lifted the Championship’s bottom side out of the Championship relegation zone.
The Hull substitute taking full advantage of loose play by Williams’ side in their own half driving infield to fire the ball into the far corner of the net to earn all three points for the home side.
That came after Harry Darling had headed the Swans back on level terms from Josh Tymon’s cross just before the interval, cancelling out Joao Pedro’s opener.
Both sides also hit the woodwork, with Ben Cabango’s close-range header striking a post for the Swans.
Ruben Selles had looked like being denied a first Hull victory in his third game in charge, but Hull were the better side throughout and the Swans, who produced arguably their poorest performance of the season, were deservedly beaten when Burstow picked off Lawrence Vigouroux’s loose pass before beating the goalkeeper from 20 yards.
Williams had no excuses and felt his side didn’t deserve to take anything from the game.
“We were poor today, we didn’t deserve to win, we deserved to lose and that’s hard to take,” said the Swans boss.
“The opposition were better than us and we performed really poorly and got what we deserved.
“I don’t quite know why we were miles off it – but we were.
“It’s frustrating but there is nothing we can do about that now; we have wasted 90 minutes of football that we are never going to get back.
“I can’t say I jumped for joy when the ball hit the back of the net [for the equaliser] because from what I’d seen for the 40 minutes before that, we were well off the pace of the game.
“I thought unless things changed drastically, we were unlikely to win the game.
“I think if we had got out of here with a point, we’d have been very fortunate. We didn’t and now we have just given ourselves a terrible start to the Christmas period.
“We have a home game now [against QPR on Boxing Day] which we have to do far better in, and we will do so we will look forward to that.”
[Lead image: Swansea City FC]