The first pre-season test was completed in Barcelona earlier this week, with the consensus Ferrari made the strongest start of the title hopefuls with just three weeks to go until the start of the season. So, we'll see at the first race.
Lewis Hamilton is not anxious by Ferrari's impressive performances during testing for the 2019 Formula One season.
Ferrari last won a constructors' championship in 2008, the year of Hamilton's first drivers' title with McLaren.
"For us it has just been about digging deep and trying to understand the auto, so pretty much the same as the beginning of every year". I think we feel at this point they might be a bit ahead, but obviously it's impossible to make detailed calculations.
Bottas believes the situation will change before the season opener as teams bring new parts, but says Mercedes can't count exclusively on that to overhaul Ferrari.
Hamilton also recognised Ferrari were quick when he spoke to reporters earlier in the week but said that was the case in testing previous year and Mercedes prevailed in the end. "I didn't feel like it fell off a cliff and we're killing it", he said.
The 34-year-old Briton enjoyed a near flawless run to secure his fifth drivers' title in 2018.
On Wednesday, world champion Lewis Hamilton, Bottas' team-mate, said Ferrari were "very strong", adding: "It appears they have a better package than previous year, which means it will be a bigger challenge for us". But there's nothing fundamentally wrong, we feel.
Valtteri Bottas has admitted that Mercedes struggled with some balance issues in their new 2019 vehicle over testing this week, even calling some of the issues "massive", but remains upbeat that any problems can be solved as long as the team keep working hard.
The Mercedes drivers haven't been up the top of the leader board for fastest time in the past three days as they have been focussing on longer stints and gathering information about the auto.
Leclerc and Vettel will once again share driving duties, getting two days of running each, before attention turns to the first race in Australia on 17 March. Hamilton joined the team in 2013, when it won just three races all year, but believes the upcoming 2019 season will be the most hard since he arrived. "We will have a better idea this time next week but even then we still won't know what fuel loads people are on and who is showing their cards and who is not".