Andre Ayew fired Swansea to a 1-0 victory against 10-man Brentford in Sunday's Championship play-off semi-final first leg as the Ghana striker recovered from missing a penalty to grab the late winner.
Ayew saw his spot-kick saved by Brentford keeper David Raya in the second half at the Liberty Stadium.
But the former Marseille and West Ham star shrugged off that blow to score with a superb strike nine minutes from full-time.
Brentford had Rico Henry sent off for a studs-up lunge on Swansea right-back Connor Roberts soon after Ayew's penalty miss.
"We're playing two games for promotion to the Premier League and 170 million," Brentford boss Thomas Frank said.
"People need to be really sure when they make decisions.
"I can't understand why we can't have VAR in these games. It was absolutely not a red card."
Steve Cooper's side will take their slender lead to Griffin Park for the second leg on Wednesday.
"It was an eventful game. I'm pleased, it's a small advantage. But it is only an advantage if we use it well," Cooper said.
"Andre brushed off the disappointment of the penalty and didn't let that affect him. That's what he does, step up in the big moments."
Swansea's south Wales rivals Cardiff host Fulham in the other play-off semi-final first leg on Monday.
Brentford and Swansea have taken contrasting routes to the play-offs.
Heading into Wednesday's final day of the regular season, Brentford were in with a chance of automatic promotion to the top flight for the first time in 73 years.
But Frank's team were beaten by lowly Barnsley, while West Brom's draw with QPR was enough to clinch second place above the Bees.
Swansea were underdogs to make the play-offs with just minutes left in their last game at Reading, but a late goal blitz gave them a 4-1 win.
That result combined with Nottingham Forest's 4-1 loss against Stoke was enough to lift Swansea into the top six on goal difference.
Bidding to return to the Premier League after a two-year absence, Swansea were on top for long periods against Brentford.
Ayew's first-half header hit the post before Raya smothered the rebound from on-loan Liverpool forward Rhian Brewster.
Brentford's Said Benrahma squandered his side's best chance when he blazed over.
Swansea won their penalty in the 63rd minute as Ayew found Brewster and he was brought down by Brentford captain Pontus Jansson.
Ayew tried to fool Raya with a stutter-step before his penalty, but the keeper stuck his hand out to save.
Just moments later, Henry saw red for his foul on Roberts and Ayew made Brentford pay in the 81st minute.
Conor Gallagher dinked a pass to Jay Fulton in the Brentford area and he laid off for Ayew to smash a half-volley into the roof of the net.
The 30-year-old's 18th goal of the season will prove priceless if Swansea can seal their place in the Wembley final.