In a wet FP2 at Suzuka, George Russell led a Mercedes one-two, 0.2s clear of Lewis Hamilton, with the #63 Mercedes 0.8s clear of the lead Red Bull of Max Verstappen.
The Dutch driver complained of overheating his tyres while on push laps at Suzuka, while Mercedes - traditionally kinder on their tyres in 2022 - did not report any such problems.
Despite the large gap between the leaders and the #1, Horner was not concerned, believing Mercedes' tyre usage pointed to the deficit.
Tyres behind Red Bull deficit
"This track is very tough on tyres," Horner explained to Sky Sports F1.
"We only ran one set of Intermediates in that second session, I think Mercedes ran ran three, so our pace is reasonable.
"It's difficult to draw too many conclusions, because you can see the front tyres are going off very quickly around here.
"There's a lot of data for the guys to be looking at. But on the whole, it was a pretty reasonable day.
"Definitely taking those extra sets of tyres, particularly in the first sector, there's a big advantage on a new set here.
"So today's times probably aren't that representative, but Mercedes are definitely in the mix, definitely looking competitive.
"I think for Ferrari are a bit out of position as well, so I think the whole thing will concertina up a bit."
Viewed by others:
Horner on difficult compromises
Saturday is expected to be dry at Suzuka, whereas Sunday is expected to be wet, forcing teams into compromises on set-up for Qualifying vs the race.
It is a challenge for the teams, as Horner explained.
It's trying to find that compromise, because tomorrow looks dry," Horner said.
" And Sunday looks as if it may be wet, so it's trying to ensure that you've got somewhere near a halfway house.
"So that you've got a reasonable amount of downforce on the car, and you've picked a level that's reasonable for both conditions.
"It's those debates that will be going on tonight to say: 'Where do you put the favouritism towards? Is it grid position, or is it the actual race?'"
F1 Podcast: Should Perez’s Singapore penalty have been decided during the race?
RacingNews365.com F1 journalists Dieter Rencken and Michael Butterworth discuss the key topics from the Singapore Grand Prix, including whether Sergio Perez's punishment should have been decided during the race rather than after.