"I'm unsure what to expect from myself swimming so many events, but I'm learning to pace myself," said Hackett, who opened the championships with a victory in the 400m freestyle.
"Every race has been hard, but I put myself in this position, and I'm determined to give it my all."
The undisputed king of distance freestyle, Hackett is the defending 800m world champion, and owns the second-fastest time in history behind world record-holder Ian Thorpe.
The 25-year-old clocked 7:47.62 to top the qualifying times, then was whisked off for a post-preliminary drug test.
American Larsen Jensen finished second to Hackett in their heat, posting the second-fastest qualifying time of 7:48.89. Yuri Prilukov was third-fastest in 7:51.75.
"It's good to be in a heat with Hackett, you know you're going to make the final," Jensen said.
In other heats, American Katie Hoff followed up Monday's victory in the 200m individual medley with a solid performance in the 200m free, posting the second-quickest time of the morning behind Italian Federica Pellegrini.
Defending world champion James Gibson was one of four British swimmers who moved into the next round of the 50m breaststroke.
Gibson maintained his grip on the 50 metres breaststroke title he claimed in Barcelona in 2003 when he qualified second fastest for the semi-finals in a time of 27.73 seconds.
Fellow Briton Chris Cook made the cut in the same event, with Welshman David Davies and Loughborough's Melanie Marshall also safely negotiating races in the morning.
Gibson had no illusions about the challenge facing him after finishing 24 hundredths of a second behind America's Mark Gangloff, with France's Hugues Duboscq in third.
He said: "It was not bad. I was a bit shocked to see the times for the 100 metres yesterday it just shows that world swimming has moved on. Some of the big guys are saving some. I will give it a good shot out there. I'm here to defend my title and I am not going to let anyone take it away."
Cook qualified in 28.07secs, a tenth of a second outside his personal best.
Meanwhile, America's Jessica Hardy set her world mark in the semi-finals of the 100m breaststroke on Monday, and will try to back up that performance in a final that includes former world record-holder Leisel Jones of Australia and China's Luo Xuejuan the two-time defending world champion and Athens Olympic gold medallist.