Sebastian Vettel posted the fastest time on an "unbelievable" first day of Formula One testing, while Williams do not plan to take part until Wednesday with their car not ready.
Ferrari driver Vettel topped the timesheet with a lap of one minute, 18.161 seconds in the first session on Monday, before going on to complete a total of 169 circuits – the most of any driver – in Barcelona.
McLaren's Carlos Sainz Jr was 0.397secs slower to surprisingly finish second-quickest, with four-time champion Vettel overjoyed by his start to 2019.
"We couldn't have hoped for a better day. I think it was unbelievable. The car was working really well, we had no issues slowing us down. We did the program just the way we wanted and we were able to squeeze a little bit more out even," he said. "The car is working. I feel comfortable. I certainly got enough laps today, I'm going to sleep well. The guys are very happy.
"It's very early, it's the first day, and it's meaningless in a couple of weeks, but for now, huge compliments to everyone in the factory, how they have tackled the new regulations. What they put on track today is very close to perfection for the first day of driving – the amount of laps we did, how the team was handling the car."
After being forced to cancel a pre-test run-out with their car still at their Grove headquarters, Williams confirmed they would not be on track in Barcelona until Wednesday at the earliest.
Team principal Claire Williams said: "It is looking more likely than not that we will now not be in a position to run on track until Wednesday at the earliest.
"This is obviously extremely disappointing, but it is unfortunately the situation we are in. We will be getting the FW42 on track as soon as we are able."
Valtteri Bottas and defending champion Lewis Hamilton placed eighth and ninth respectively, but Toto Wolff held little stock in the standings after one day. However, he confessed to being very impressed with Ferrari, who ran Mercedes closest in 2018.
"You need to be disciplined. We are all keen and looking at lap times and obviously you want to be quick and on the top of the leaderboard at every single session, but it's not the purpose of the test," he said. "I've seen the time and the time was very quick, two seconds quicker than everybody else, and definitely the Ferrari has been going strong this morning
"We feel no sense of entitlement, and today shows you why. The Ferrari looks ultra-strong."
WHAT THEY SAID
Red Bull's head of race engineering Guillaume Rocquelin: "No major problems. We were a bit late getting out this morning due to a frustrating little niggle but we got that sorted out without any real fuss and once Max got into his stride, we enjoyed a very good first day."
"Today was about bedding ourselves back into it and, for me, continuing to learn about a new car and a new team. It was a decent afternoon and it's important we keep increasing our mileage. We have plenty to analyze but it's been a promising first official day. I'm excited to hit the track again and develop this new relationship," said Renault driver Daniel Ricciardo.
Kimi Raikkonen after bringing out the red flags early on in his Alfa Romeo: "Obviously we started the day off with a spin ... but apart from that, everything ran smoothly. The first feelings are very, very positive. I think we have a car we can work with."
Sainz on a positive day despite his car stopping in the pits near the start of the second session: "It was just a very slight cable that was not so connected, so nothing reliability-wise, nothing to worry about, just a car build-up issue. So something to be very proud of as a team."