"I can truly say something in my mindset has changed," Bottas said. "The preparation over winter and just the way I feel and think about things changed. It's quite difficult to explain in detail, but I feel different."
But Hamilton has not noticed except for the obvious change.
"He's the same guy he was last year. There's no difference for me except he has a beard," Hamilton said. "I've never thought he was a weak person, anyway, I've always seen him as a strong character."
Hamilton has won four of the past five F1 titles driving for Mercedes, missing out in 2016 when he finished second to teammate Nico Rosberg. But he insisted the loss in Australia — and the prospect of another strong in-house challenge — doesn't faze him.
"Not really. It doesn't really change a thing. We move on," Hamilton said. "It was a great race from him and I look forward to seeing more from him."
He also expects Ferrari to bounce back after a disappointing race in Melbourne, where Vettel finished fourth and Ferrari newcomer Charles Leclerc was fifth. Both were around a minute behind Mercedes, an alarmingly uncompetitive performance considering how slick Ferrari had looked in pre-season testing in Spain.
"We expected more. I made quite a lot of mistakes in qualifying," Leclerc said. "We expect to be more competitive [in Bahrain]."
Mercedes boss Toto Wolff thinks Ferrari will do better on a track which encourages overtaking.
"We've seen the potential of Ferrari's package in Barcelona, so we expect them to come back strong," Wolff said. "After the Barcelona test, we felt we were the challengers. Whatever the Melbourne result says, our mindset hasn't changed."
Bahrain's 5.4km track sits in the middle of the desert, on a former camel farm. Although desert winds sometimes sweep sand on to the circuit, it quickly dissipates and causes little disruption.
Temperatures can hit nearly 40 degrees Celsius in the afternoon, but the circuit is considerably cooler when the race starts at 6pm local time. Qualifying is also at that time but two of the three practice sessions are in the searing afternoon sun and less reflective of how cars are managed for the race itself.
- AP