Lewis Hamilton believes Mercedes and Ferrari are neck and neck with their upgraded engines at the Belgian GP despite the Scuderia's perfect start to the weekend.
Ferrari completed a Friday practice double for the first time this season as Sebastian Vettel and then Kimi Raikkonen made the most of the team's new 'Spec Three' power unit.
But Hamilton was impressed with his team's own engine development, improving in Practice Two to finish a tenth of a second adrift of the session-topping Raikkonen.
"We've brought a new engine and everything was running smoothly," the championship leader, who claimed he had "missed driving" over the summer break, said.
"It's really close between us and Ferrari. It seems like we've both been bringing new updates at a similar rate."
Practice Two: Raikkonen just ahead of Hamilton
When's the Belgian GP on Sky F1?
Vettel added: "Today it's a bit difficult to know [how well it's working].
"For now, I believe everybody seems quite confident and quite happy that the engine and power unit is doing what it's supposed to."
And Raikkonen continued: "We never put anything in the car that we don't think is a step forward. But it's not a night and day difference."
It wasn't all plain sailing for Vettel, who is aiming to cut Hamilton's 24-point title advantage this weekend.
The German was fifth and well over half a second off the pace in Practice Two as Raikkonen impressed in the qualifying simulations on the supersoft tyres.
"I'm not quite where I want to be," Vettel admitted.
"We have a bit of a margin, there's quite a bit more in the car so I need to get to it tomorrow. Here it is important to feel confident to get the right timing because there's not much time to think.
"It just needs to click and today over one lap it clearly didn't, but overall I think it's going in the right way."
Red Bull off the pace
It was a mixed day for Red Bull. Max Verstappen was only a tenth off Vettel in Practice One, though Daniel Ricciardo was merely a spectator of the session after suffering a fuel issue.
Then, in the final session, both drivers were more than half a second off the pace.
"We have to be realistic, I think fifth and sixth is the best we can do here," stated a frustrated Verstappen. "We are so much down on power, that's the big problem.
"I think we can still find something on the car, but it's not going to change to pole position. We just hope we can be closer in the short run because in the long run it didn't look too bad."
Ricciardo added: "I'm not sad, but we can do better."
The Australian also spoke about his first day back on track with the team after informing Red Bull he was leaving for Renault next season.
"I had a good bit of team time yesterday and spoke to everyone," Ricciardo told Sky F1. "Everyone's cool and everyone respects the decision.
"I think more than anything, everyone's ready to kick on and finish the year strong."
F1's back from its summer break with the Belgian GP this weekend, live on Sky Sports F1. The race begins at 2.10pm on Sunday. Get Sky Sports F1 - the home of every race weekend LIVE in 2018.