"Surely in the second session I could have driven a little better," the 38-year-old former world champion told reporters.
"I went to the curbs in the fastest lap, so we lost a lot of time, but overall it was pretty easy."
Championship leader Hamilton was only 0.011 seconds behind Raikkonen, after using the fastest hyperspace tires for the night session at the spectacular Marina Bay street circuit.
Mercedes, who usually struggles for pace in Singapore, chose not to use the fastest compound in the first session, in which Hamilton finished sixth.
"We were close to the Ferrari, but we'll only know tomorrow how fast they really are," said four-time world champion Hamilton, who finished more than half a second ahead of Max Verstappen's Red Bulls and Daniel Ricciardo.
Vettel limped back to the pits only the ninth fastest with fluid coming out of a hydraulic pipe after it bounced off the wall and damaged his right rear and front wheels with 40 minutes of the remaining session.
It meant that the German, who is 30 points behind Hamilton in the standings with seven races ahead, could only complete 11 laps under the lights in race simulation conditions.
But Vettel denied that the setback would impede him in Sunday's race.
"No, not at all," he told reporters. "For now I think you have enough experience reading the others, what they did and reading their careers with tires that obviously will be key for Sunday, but we can recover most of it tomorrow."
Valtteri Bottas was fifth in the second Mercedes, less than a tenth of a second behind Ricciardo, with the Renault of Carlos Sainz sixth.
Singapore's former specialist, Ricciardo, who has finished second in the past three consecutive years, led Red Bull one-two in the first practice.
But Verstappen said he did not think the Red Bulls had a package fast enough to challenge Ferrari and Mercedes in pole position in Saturday's qualifying.
"I think we can still improve with the car," said Verstappen. "I do not think we have the pace to fight for pole, but we will try to optimize the car a bit more."
The new signing of Ferrari, Charles Leclerc, had a moment to forget when he crashed his Sauber near the end of the first session.
The 20-year-old from Monaco, who will replace Raikkonen in the Marinello team in 2019, miscalculated his exit and cut the wall hard at Turn 13, destroying the right front wheel and suspension.
The Marina Bay track has 23 corners, more than any other in the current Formula One calendar, and is a severe test for both drivers and cars, with temperatures that regularly exceed 30 degrees Celsius during the night race.
"The track is incredible," said Hamilton. "We are much faster than last year, but that also makes the return much more difficult with the G forces and it's very hot, and in the second session, I think I lost almost two kilos."