The Zenit midfielder admits it would be something of a "miracle" for the Red Devils to triumph in Brazil but insists they believe that they can make their "dreams" come true.
The Red Devils booked their place in the last eight with a dramatic 2-1 extra-time win over United States in Salvador, just hours after the Albiceleste had scraped through thanks to Angel Di Maria's 118th-minute winner against Switzerland.
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Witsel says the pressure is now off Belgium, which has only once made it to the semifinals of the World Cup, and that Marc Wilmots' men will, therefore, have no qualms about taking the game to Lionel Messi and company.
"Our goal was to reach the quarterfinals," the midfielder told reporters in Salvador. "Now we are here, we can enjoy the next game and I think we have a chance to get through to the semis. Argentina are a very good team, with top players like [Lionel] Messi, Di Maria and [Gonzalo] Higuain.
"But we have quality too. We're not afraid of Argentina and we'll go out to win the game. Everything that happens for us now is just a bonus. I don't know if we can perform a miracle here by winning the World Cup but if you just take things game by game, you can make your dreams come true."
Indeed, Witsel is hoping that anything is possible for a side that exhibited great perseverance and resilience in defeating USA 2-1 at Arena Fonte Nova.
Belgium peppered Tim Howard's goal without shots for 90 minutes but only made the breakthrough in extra time, going two goals to the good thanks to Kevin De Bruyne and Romelu Lukaku.
However, the Red Devils were left hanging on after Julian Green pulled a goal back for the U.S. and Witsel admitted he was a relieved man at the full-time whistle.
"It was a frustrating game for us," the Zenit star confessed. "We had so many chances to score.
"We had 38 shots on goal. We shot and we shot but the ball didn't want to go in. So you start to think that maybe USA will score on the counterattack. But we gave everything. We kept going and thankfully we won."