Max Verstappen confirmed that a loss of power was behind him lagging over a second off the leading pace set by Ferrari during practice for the Belgian Grand Prix.
Verstappen was 1.2 seconds adrift of the best time set by Charles Leclerc, reporting during the session that engine braking issues, which have popped up at a few races this year, had returned.
However, the Dutchman expects that he will be able to pull much closer to the red cars come qualifying, although doubting that they can be beaten around the fast Spa-Francorchamps.
Verstappen told Sky Sports: "The car actually didn't feel too bad, I think were quite competitive in the second sector.
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"But the whole day I couldn't use my normal power, so we had to drive a bit on low power the whole day to what we normally use on the Friday.
"So that's not ideal around here and that's why I think we are a bit further off at the moment but I think it should be better tomorrow when we put the other engine in.
"I don't think we can beat Ferrari but I think we can be close."
Asked if he might be worried about reliability in the race, Verstappen replied: "No it's not a concern.
"We knew it was coming up, but obviously you have your few engines you can use and of course some of them getting a bit older so you also have to think about the mileage."