Denmark secured three valuable points with a 1-0 win over Peru in Saransk but were under pressure for much of the game.
They now head to Samara knowing a win over Australia will put them in the driving seat to qualify from Group C.
Tottenham Hotspur midfielder Eriksen, who provided the assist for Yussuf Poulsen’s winner, admitted Denmark didn’t produce the attacking football that yielded 20 goals in qualifying.
"We certainly weren't at our best," Eriksen said after the win over Peru.
"Peru were well organised and they made it hard for us. But it is not always necessary to play amazing football to win.
"You don't get many chances at a World Cup and it was nice to see that we can actually also win even though it wasn’t great to look at.”
Denmark will be without midfielder William Kvist, who sustained suspected fractured ribs in the first half against Peru. He will likely be replaced by Ajax’s Lasse Schone against the Socceroos.
The Socceroos thrive in the role of underdog, and this was certainly the case in their 2-1 loss to France in their Group C opener, when they went down after conceding a penalty and an own goal.
They won't be the favourites in Samara, either, but after coming so close against France, coach Bert van Marwijk only had positive words for his players.
"One of the most positive things is that we showed guts and we could be ourselves. The next game is another game, but it gives us confidence," he said.
His players have also bought into the message in the wake of the loss.
"The team had tremendous guts, we just couldn't get ourselves over the line," said defender Trent Sainsbury.
"We were almost perfect."
Van Marwijk may be tempted to turn to veteran Tim Cahill, looking to score at his fourth FIFA World Cup, or Daniel Arzani, the youngest player at the tournament, for further attacking options.
A win for Denmark would all but secure a place in the last-16 for the first time since 2002, depending on the result of France against Peru.
Australia, who are playing in their fifth FIFA World Cup finals, need a positive result to maintain realistic hopes of qualifying for the last-16 for the second time in their history.