Messi has scored four goals at this World Cup, all in the group stage, and the Argentina captain will be their key man in the Maracana as they look to win their first title since 1986.
But Wenger, who is in Brazil doing television work, does not believe the Barcelona forward has been at his very best.
"Messi looks to be motivated and although he didn't look like that in South Africa, this time he looks on board and I like the look of him," Wenger told beIN Sports in an interview reported in the Daily Mirror. "Having said that, he still hasn't reached his top gear.
"Compared to the Messi I know, he is a fraction of a second behind his usual self -- I don't know if that's anything to do with the fact that he was injured many times last season and he hasn't been able to let it go completely. But he's just not in the same place physically as before. However, despite this, everything else is there with Messi and I have never seen him so extrovert emotionally as he is now with the national team."
Wenger believes it is the big players who have made a difference throughout the World Cup.
"You need players that make the big decisions," he added. "When everything is going well then you just need your team performance to be focused. But when things are going badly then you need a guy who can take the ball and create something special.
"I ask you now, would Argentina be in the World Cup final without Messi? Would Netherlands be in the semifinals without [Arjen] Robben? You would certainly say no. Would Brazil have gone through the group stages without Neymar? Certainly no.
"That means the big players are important because they can make something special on their own. Also, when you sit in the dressing room before a big game and you look around you and you see superstars like Messi and Neymar then you always know you have a player that can deliver something and it increases the belief of the team as a whole. Even just their presence sometimes is very important in these kind of highly pressurised games."
Wenger was also full of praise for 21-year-old France midfielder Paul Pogba, who has been nominated for the young player of the tournament award.
"Paul Pogba is a world-class player potentially and I think he has absolutely every quality that every footballer could dream to have," Wenger said. "He has the power, the charisma, the technique and he has this incredible acceleration and pace.
"To see him at high speed during France's games was absolutely amazing and he is the fastest player for France. So you would think he could take it all the way.
"I think the only question mark is whether he really wants it mentally. He will answer that question certainly in the coming years but potential-wise I think he is the best midfielder of tomorrow in the world."
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