Spielberg - Championship leader Lewis Hamilton said he expects a "real challenge" to fend off his rivals after winding up fourth in Friday's action-packed opening practice sessions at the Austrian Grand Prix.
The defending five-time champion warned that Ferrari demonstrated the speed and potential to control Sunday's race on the fast and compact Spielberg track, with its swooping corners, in the Styrian Alps.
"I think today has shown that they're either with us or even slightly ahead of us," he said, referring to Ferrari, after Charles Leclerc had topped the times for the Italian team.
"It's one of those circuits where it's very, very close. If you go, for example, to Brands Hatch, it's the small tenths that make a difference - so it's quite packed at the front.
"I think the Ferraris have generally been very quick on the straights, the Red Bulls have picked up their pace with their new engine. I think they're doing great.
"So, it's definitely going to be a real challenge."
Mercedes have won a record eight consecutive season-opening races and 10 in a row overall, but may face a fight to score a record-equalling 11th successive victory.
"Ferrari have been quick all year on the straights and it doesn't seem to be hampering them in the corners here," he added. "So, it will be interesting.
"On race runs, they are not usually so strong, but today they were quicker than us... But that's ok and we will keep fighting.
"This is a track where you can follow closer, at least, and hopefully the tyres are durable enough for us to do that.
"I don't know if we're faster than we were here last year, but it feels incredible. You're flat out for a large portion of this lap.
"You've obviously got a couple of slower corners like one, three and four, but otherwise it's super-fast. Turn six, seven is like a small lift, nine.... It's ridiculous how quick we're going through nine and then the last corner which drops away."
He added that he and team-mate Valtteri Bottas who was second fastest behind Leclerc had been "tickling" with their cars' set-up all day to try and improve it.
Bottas was one of two drivers to have a serious crash - Max Verstappen of Red Bull was the other - during a chaotic afternoon session when a gusty wind caused havoc.
"It was a pretty big hit for me," he said. "But I'm ok and without any pain. I must say I prefer this kind of circuit, which penalises mistakes and makes it even more exciting."
Like Hamilton he warned of the likely affects of the European heatwave.
"It's going to be very hot, especially on Sunday, and that will test the cars to the limit in terms of cooling the brakes and the engine... It's going to be very close."