Wales backed up last weekend's thrilling win against tournament hosts England to win as tries from Gareth Davies and Scott Baldwin as well as 13 points from Dan Biggar's boot left Warren Gatland's men top of Pool A.
However, the group remains wide open with England - who did pick up a bonus point against Fiji - playing Australia at Twickenham on Saturday.
Wales now sit on 13 points, four clear of the Wallabies (nine) who face England (six) at Twickenham on Sunday (AEST).
An Australia win will put Wales into the quarter-finals, but should England beat the Wallabies they will be favourites to go through as their last pool match is against Uruguay, while Wales face off against Australia.
But Warburton insists Wales' primary focus was to just get the job done against Fiji, who exit the competition.
"Our priority was to win," he told ITV.
"Australia didn't get a bonus point against Fiji either.
"Credit must go to Fiji. It was so tough. I was knackered by the end of the first half.
"They played like they had nothing to lose, it was a quick game.
"But we have three wins out of three now and if someone told us that before the tournament we would have bitten their hands off."