The world champion was 0.704 seconds slower than Mercedes team-mate Nico Rosberg's pole lap and was also behind Red Bull's Daniel Ricciardo.
"It is a track where you need a real good rhythm and I haven't been in a rhythm all weekend," Hamilton said.
"I couldn't hook up the laps, couldn't find grip, which is unusual for me."
Hamilton, who starts the race two points ahead of Rosberg in the championship, said: "I will see how it goes. If it goes well, it does. If it doesn't, it doesn't."
Hamilton lost half of Friday's second practice session because of a hydraulic leak and when he was on track had a number of incidents in which he locked his front wheels and ran wide.
But Mercedes said there may have been a problem with the car causing his difficulties - their information said it was producing less downforce than Rosberg's and they were to analyse the situation further overnight ahead of the race.
Hamilton made a reference to the number of problems that have afflicted him this season, including the technical problems that were partly responsible for him falling 43 points behind Rosberg after the first four races.
He said: "I've kind of been on the back foot all year long so there's no real difference to me.
"I'll do the best job I can from there and try and fight my way through. It's a very hard track to overtake, so you're usually stuck where you are - but there's lots of safety cars at this race so lots can happen during the race."
Rosberg said he was wary of the threat posed by Red Bull's Ricciardo in the race, which starts at 1300 BST.
"For sure he is going to be a threat," he said. "I'm not worried but for sure I will be keeping an eye on him, because they were quick on race pace."
Red Bull were faster than Mercedes when the teams did their main race-preparation work on full tanks of fuel in second practice on Friday.
The threat from Red Bull is exacerbated by the fact that Ricciardo and team-mate Max Verstappen, who qualified fourth, will start the race on super-soft tyres while Mercedes will be on ultra-softs.
It remains to be seen which choice turns out to be the best in terms of strategy.
"It was always the plan, even before the weekend," the Australian said. "We expected other teams to do it as well. I would say we are surprised we are the only ones."
The Singapore race has been running since 2008 but has been won by only three drivers - multiple world champions Hamilton and Fernando Alonso, with two victories each, and Sebastian Vettel, with four.
Rosberg said: "It may be time to change that then. I was surprised to hear that, actually. But anyway, I'm feeling good for the race and, of course, I have a good chance."
Ricciardo, who has already missed out on two wins this season after team errors in Spain and Monaco back in May, said he was keen to grab what might be his last chance of a victory this season.
"Obviously I would love to win it. I think we've created a good opportunity, starting on the front row. I think as Nico touched on, our race pace was pretty good yesterday.
"Here track position can dictate a lot and safety cars as well. We're in a good position."
Singapore Grand Prix qualifying results
Singapore Grand Prix coverage details

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Lewis Hamilton blamed himself for his struggle to third on the grid at the Singapore Grand Prix, saying: "I just haven't been quick this weekend."