Jamie Paterson's stunning goal sealed Swansea's comeback to beat West Brom 2-1 as the Baggies lost ground on Championship leaders Bournemouth.
Karlan Grant's goal after just 52 seconds gave the Baggies a deserved 1-0 half-time lead, but Swansea were the better side after the break.
In-form striker Joel Piroe's eighth goal in all competitions got them level and then the Dutchman set-up Paterson who finished with aplomb.
West Brom were ahead straight from the kick-off. Paterson - Swansea's South Wales derby hero against Cardiff on Sunday - lost possession inside his own half and it proved crucial.
Matty Phillips and Callum Robinson took full advantage and it was the latter who squared for Grant to lash the ball home as the Baggies went 1-0 up in clinical fashion.
Swansea's poor start continued with a short corner routine which went awry and, while Semi Ajayi was booked, West Brom were well on top with the wet weather suiting their slick passing.
Baggies captain Jake Livermore also blazed over as the high early press of the visitors put Swansea's passing game under pressure.
Slowly, Russell Martin's side began to find their groove and Piroe had their first effort on goal with a tame effort from the edge of the box.
Swansea head coach Martin and his players grew increasingly frustrated with referee Simon Hooper as the first half wore on and Phillips worked home goalkeeper Ben Hamer.
Some of West Brom's tackling was physical at best and Jayson Molumby was perhaps fortunate to escape a red card for a horrible sliding challenge on Swansea captain Matt Grimes.
Martin introduced Oliver Ntcham for Flynn Downes for the start of the second half but West Brom defender Matty Clarke headed wide on the resumption.
Swansea's bench continued to make their ire with Hooper clear to such an extent that the Welsh side's goalkeeping coach Dean Thornton was booked.
It came after Piroe was flagged offside in what looked like a marginal call. Not long later, Paterson clipped another long ball forward to Piroe.
This time the Dutch striker looked well offside, but the flag never arrived and Piroe held off Clarke to finish in cool fashion.
Ntcham then came close to a second as Swansea started to click.
Paterson put a 25-yard free-kick into the away wall and Ntcham hit the side netting. Phillips also unleashed a wicked shot which clipped the bar at the other end as the game opened up.
Paterson had a shot deflected over and Ntcham made a big difference to Swansea's attacking threat, but there was still time for Swansea to find a winner.
Piroe played in Paterson who jinked past former Swan Kyle Bartley in sublime fashion to finish and send the Swansea crowd mad for the second time in four days.
What the managers said...
Swansea's Russell Martin: "It wasn't an ideal start and we spoke to the guys about the importance of starting well against a team like West Brom who have a lot of quality. The way we reacted was incredible really. We played really well and with a lot of intensity, especially in the second half against a team who are very hard to play against. I think we deserved to win. We wanted to back up the derby performance with another one and thankfully we did that.
"We joke with Joel that he needs to calm down in front of goal, but he doesn't feel any pressure. He possesses the qualities of all the top players and it's incredible really for a player of his young age. When he's involved, he's involved in key moments and it's the same with Jamie. We have started slowly but the performances have got better and in the last two games we have added quality and composure in the final third. We have had two big results and performances."
West Brom's Valerien Ismael: "We are disappointed to lose the game. We started well and had a good forward press which forced Swansea to play a lot of long balls. Our problem throughout the game was our transition and we lost the ball too quickly. This was the reason Swansea got back into the game. We had a big chance to take the lead after Swansea scored with Matty and the goalkeeper made a big save.
"We don't need to overreact because we play our next game on Saturday at home. We need a reaction and the right mentality. The Championship is a long season and every team will struggle at some point because every team is of such high quality."