Guy was the 400m freestyle silver medallist in Kazan two years ago, but was sixth at the Duna Arena as China's Sun Yang won a third straight title in the event.
Sun clocked three minutes 41.38 seconds to reverse the Rio standings as Olympic champion Mack Horton of Australia had to settle for silver in 3mins 43.85secs.
Olympic bronze medallist Gabriele Detti of Italy was third in 3:43.93, while Guy, who was sixth in Rio, finished in 3:45.58.
"I came in and thought, '3:45, is that it?' I don't know what happened," said Guy, who begins the 200m on Monday's second day.
"I thought I'd go faster than that. I thought I'd go 3:43 mid to high, based off this morning's effort. It's an okay swim.
"This morning (in the heat) I felt strong and hopefully I can deliver in the 200."
Horton and Sun have history, with the Australian unhappy at the perceived lenient punishment given to the Chinese for an anti-doping infringement.
Park Tae-hwan of South Korea, the 2008 Olympic champion and another with a doping ban on his record, was fourth in 3:44.38.
Guy hinted at frustration at Sun retaining his title.
"It is Sun Yang and he's got the body for it and everything," Guy said of Sun's 3:41 swim.
"I can't say too much on him because of the past. He's won it and that's it really, isn't it?"
Ben Proud qualified for Monday night's 50m butterfly final in fourth place.
The Plymouth sprinter clocked 22.92 as Caeleb Dressel of the United States advanced quickest in 22.76.
Olympic champion Adam Peaty qualified quickest for Monday night's men's 100m breaststroke final.
The 22-year-old won Great Britain's first gold medal of the Rio Olympics in a world record 57.13 seconds - 1.5secs ahead of his nearest rival.
Peaty was 0.93secs clear of his rivals in Sunday morning's heat, clocking 58.21, and was even quicker in the semi-finals at the Duna Arena on the banks of the Danube on Sunday night.
Peaty, the 2015 world champion in the event and over 50m, clocked 57.75 for a new championship record.
Kevin Cordes of the United States was second quickest in 58.64, while Scot Ross Murdoch advanced to the final in seventh place in 59.23.
South Africa's Cameron van der Burgh, the London 2012 gold medallist and Rio runner-up, was absent from his heat, with his lane left empty. The London 2012 gold medallist is concentrating on the 50m event.
