Kenny Jackett has warned his Millwall side they must be at the top of their game if they are to reach the League One play-off final.
The Lions claimed a first-leg advantage in their semi-final clash with Leeds as Neil Harris' 71st-minute strike gave them a 1-0 win at the New Den.
Millwall managed to restrict Leeds' 52-goal strike partnership of Jermaine Beckford and Luciano Becchio to few chances and Jackett knows the Lions need a similar effort at Elland Road on Thursday to stay on course for promotion.
"It's all set up and we know we've got to go to Yorkshire and play right to our maximum if we want to go through," he said.
"These play-off games are very tight and this one is no different.
"It was a very good clean sheet. The back four kept a very good Leeds frontline quiet and it was key.
"There's no doubt that today has helped us slightly but we need a big performance to go through to Wembley."
Jason Price was preferred to Harris in the starting line-up but the club's all-time record goalscorer was called into action early on after Price suffered a hamstring strain.
And Harris proved to be a constant threat for Millwall until pouncing with a 12-yard strike after Richard Naylor failed to cut out Gary Alexander's cross.
That gave Harris his 117th goal for Millwall and sealed the club's first-ever play-off win in nine attempts.
"It's a goalscorers goal," Jackett added. "I'm pleased to see he kept it low.
"Maybe it was written. It was a terrific goal."