The 65-year-old said that his players could ill afford to trip in the Group A clash at Sao Paulo's Itaquerao stadium due to the resultant pressure in later matches, Xinhua reports.
"We need to win the first match to allow us to possibly slip-up in the second or third and still progress (to the knockout phase). Our tactical work has come a long way. There is little more to do before the first match," he said.
For Brazil, Thursday's clash will be followed by fixtures against Mexico June 17 and Cameroon June 23.
Having slipped to 22nd spot in the FIFA world rankings in June last year, the Selecao are now third following 15 victories from their past 16 matches.
Included in that run was last year's Confederations Cup triumph, sealed with a 3-0 demolition of world and European champions Spain in the final.
But Scolari refused to bask in past glories as he underlined the importance of a positive start in Sao Paulo.
"What worries me is Croatia. It's important that we start the match well and pressure the opposition from the outset.
"When we score early it seems that everything falls into place. It makes the opponent change their strategy and often lose their way," he added.