Gabriel Mercado's strike helped Argentina to a hard-fought 1-0 win over their bitter rivals in Melbourne, with Brazil missing a number of fine chances to equalise after the break.
Messi was unable to take his side by the hand in a tightly contested encounter, but Sampaoli was not interested in criticising his captain.
"When it comes to Messi, he is and will continue to be the best player in the world until he retires," Sampaoli said at a news conference after his first game in charge of the national team.
"That is simply the way I feel about Leo.
"Overall, I appreciate the enthusiasm and the attitude of the players. We did not have a lot of time to prepare.
"We got more chances to score than Brazil in the first half and were mainly playing inside their half. We suffered a bit after the break. Brazil got some chances to level the scoring in the second half.
"It is always a motivation to beat Brazil, this game is a classic. We did not just come here to play, but wanted to win. The players fought for the jersey."