Lees, who coached New Zealand when it was defeated by Pakistan in a gut-wrenching semifinal match at Eden Park in 1992, said the Black Caps needed to play to their best and get on top early.
"I think the key is momentum.
"You've got to get on a roll, bowl a couple of maiden overs and then put the pressure on them [Sri Lanka]," Lees said.
"If New Zealand don't win, then everyone will say OK they did all right, it's time for them to come home. But if Sri Lanka lose, then their houses will get burnt down. The pressure is all on them.
"It was the same when we played South Africa. When [Jacques] Kallis got out, that led to lesser players to start thinking they might struggle to score runs too. You can choke because of pressure."
Lees said it was similar in 1992 when New Zealand captain Martin Crowe limped out of the semifinal against Pakistan at Eden Park after he pulled a hamstring.
"Then panic went through the team, and we were pretty weak without him."
Lees also played for New Zealand in a World Cup semifinal loss - in 1979 against England.
New Zealand has made a semifinal at a World Cup five times.
Lees said two batsmen and two bowlers had to stand up for New Zealand.
"In previous games we've had one batsman who has been able to rescue us. But we need to get two batsmen playing well. If one of them gets a 100 and the other an 80, then the rest only have to get 100 between them and we'll be in with a chance ...
"I think Brendon McCullum owes the team a big score."
Lees, though, said he was a realist.
"It's going to be very tough. Sri Lanka are probably now the favourites to win the whole thing. My head says it is 70-30 for them to win. My heart says 50-50, and if we won it would be great for the game and the country and all those nice things.
"And you never know. Things can go your way. That is the nature of one-day cricket."
