The Brit won his fourth Drivers’ Championship last year with two races to spare and he is out to defend his crown this time around.
Hamilton is expecting a tougher season this time around as Ferrari and Red Bull have made plenty of improvements to their cars.
Day one of testing in Barcelona took place yesterday after a four-day break because of the bad weather conditions.
It was Sebastian Vettel who recorded the fastest time, ahead of Hamilton in second.
Mercedes split their track time between Hamilton and Bottas with the former racking up 86 laps in the morning before the latter drove 91.
However, after the session Hamilton took to Twitter to reflect on his 2017 success and jeer up his supporters ahead of the new season.
“This photo was taken before the 2017 season started,” Hamilton wrote, accompanied by a darkened picture of himself.
“Before the battles, the wins, the defeats, the lows and the highs that led to my 4th World Championship.
“We never truly know what the future will hold but you can do everything in your power to make it yours #StillWeRise.”
The 2018 season gets underway at the Australian Grand Prix on March 23.