Lewis Hamilton believes Ferrari are strong favourites for Belgian GP qualifying, with the championship leader fearing he will not even be able to wipe out "half" of the Scuderia's eye-catching Friday advantage.
Ferrari, looking for their first victory of the season at the high-speed Spa circuit, enjoyed the perfect start to their weekend with consecutive practice one-twos, as Hamilton and Mercedes' team-mate Valtteri Bottas finished 0.8s slower than Charles Leclerc following P2's qualy simulations.
"We definitely don't even have half of that to catch," said Hamilton, who claimed Ferrari were gaining one second on the long Belgian straights.
"All the time is on the straights. There's not really a lot we can do to catch that up. They've always been quick all year long on the straights so I kind of had a feeling this weekend would work really well for them."
Valtteri Bottas, who finished third and ahead of Hamilton in the afternoon session, agreed with his team-mate as he added: "Ferrari has been really strong today and they are really killing us on the straights.
"We need to make big steps if we want to battle with them."
Mercedes boss Toto Wolff described Ferrari as the "absolute benchmark" from Friday, with the Italian team - who head into a crunch fortnight with two winnable races, including their home Italian GP next weekend - also clear frontrunners in first practice with Sebastian Vettel ahead.
Hamilton believes Ferrari can still be reeled in come Sunday - but that it may just have to be down to strategy rather than "pure pace".
"We'll just have to somehow apply the pressure another way if we don't have it on pure pace," said the Englishman.
"I think the long run was very strong. If we're quick in the race it could be good."
Ferrari doubt big advantage
Despite Mercedes' claims, Ferrari, wary of past improvements from their main rivals, believe F1 2019's leaders will be right in the mix over the weekend.
"Of course they can [catch us]," Vettel told Sky F1. "It's not the first time Friday has looked good for us. Probably Saturday morning will look good for us. Then we see what happens Saturday afternoon.
"I'm not taking anything for granted."
Leclerc said a "good slipstream" contributed to his lead in P2 - he was also 0.6s faster than Vettel - and says Mercedes will be turning their upgraded engines up on Saturday.
"For sure [they can catch up to us]," Leclerc stated. "I'm pretty sure they will.
"In the second sector they are very strong and once they turn up their engines they won't be very far off in the first and third sector."
The youngster continued: "It's been a positive day. Qualy pace was very strong, race pace a little bit less so we need to work on race pace for Sunday."
Can Red Bull get in the hunt?
On the face of it, Red Bull are well off the pace compared to Ferrari, with Max Verstappen sixth and behind Sergio Perez in P2, over a second slower than Leclerc.
Alex Albon, who impressed in the morning session, finished 10th.
But Verstappen was neck-and-neck with Leclerc on the medium tyres before suffering engine issues, and the Dutchman - cheered on by a sea of orange in Belgium - claimed he was running on lower power all day.
"The car actually didn't feel too bad," he said. "I think we are quite competitive in the second sector.
"But the whole day I couldn't use my normal power so we had to drive a bit on lower power the whole day, to what we normally use on a Friday. It's not ideal around here, and I think that's why we are a bit further off.
"But it should be better tomorrow when we put the other engine in. I don't think we can beat Ferrari but at least we will be close."
Albon, who replaced Pierre Gasly over the summer break, will start at the back of the grid on his Red Bull debut, but was encouraged by his opening day.
"It was a pretty smooth day. It is a very different [car]. It's going to take a bit of time to get used to but once you're on the track with your helmet on you can feel the car a bit better.
"But it's just baby steps really."