Barcelona’s Lionel Messi after scoring against Atletico Madrid Saturday. (AP)

Barcelona: Billed as the showdown to determine Argentina’s best player, Lionel Messi left no doubt about who deserves the tag after scoring one goal and setting up another in FC Barcelona’s 6-1 win over Sergio Aguero and Atletico Madrid.

The 21-year-old Messi scored with a free kick shortly after earning a penalty that Samuel Eto’ converted. Messi then set up the Cameroon striker for Barcelona’s fourth goal on Saturday before creating several more occasions before the half ended.

“Leo was marvellous,” Barcelona coach Pep Guardiola said. “He is a really decisive player.”

Messi nearly managed to add another impressive strike to a growing resume of highlight-reel goals in the 40th minute.

The Barcelona winger collected the ball just inside the Atletico half and held off Raul Garcia to take his run into the area. He then dribbled around defender Tomas Ujfalusi and beat goalkeeper Gregory Coupet who, watched the quick chip bounce wide of the post.

“It’s been a great day,” Messi said. “It is a shame that I could not score (with that run), but more importantly, we had a great win and I am very pleased.”

Messi’s performances this week have further cemented his reputation of being the world’s best player, with only Manchester United’s Cristiano Ronaldo able to contest.

The Rosario native came on as a substitute to score both goals — including one in injury-time — to salvage a 2-1 Champions League win at Shakhtar Donetsk on Wednesday.

Aguero had few chances to impress as Barcelona’s defence smothered the Atletico striker — and the team’s offence.

The 20-year-old Aguero leads the Champions League with three goals, but while Messi roared along with the 98,772 spectators, Aguero looked disconsolate.