Vettel ended up winning by 33 seconds to extend his lead in the drivers' championship to 60 points, but Horner still thinks that the remaining races will be close.
"There were times last weekend when Sebastian was two seconds quicker than the rest of the field, but I think it was due to unique circumstances," Horner told Autosport.
"It was a circuit he was completely on top of and the set-up of the car was working well. It was man and machine working at the top of their game.
"The next challenges we have in Korea and Japan are quite a bit different in characteristics."
Vettel has won the championship for the last three seasons.