Norris quickest in Bahrain as Hamilton calls for 'equal playing field'
With the season opener in Melbourne under a month away, this was the second chance for teams to test their new cars on the track, following the shakedown behind closed doors in Barcelona at the end of January.
Every car has undergone a radical change since the end of last season following dramatic regulation changes to both chassis and engines.
There was no stopping Norris in his McLaren on this opening day as he posted the best lap of 1min 34.669sec.
However, nothing should be read into that as teams were testing different aspects of their cars with varying fuel loads.
Max Verstappen, who will be searching for a fifth world title this season, put in the most laps with 136 as he put his new power unit, built for the first time by Red Bull, through its paces.
The Dutchman was next quickest behind Norris, prompting a concerned nod from Mercedes team principal Toto Wolff.
"They (Red Bull) are able to deploy far more energy on the straights than everybody else," Wolff was reported as saying by the BBC.
"You are speaking a second, over consecutive laps.
"On a single lap we have seen it before but now we have seen it on 10 consecutive laps with the same kind of straight-line deployment."
The Austrian reportedly said that on this first day of testing Red Bull had "set the benchmark".
Wolff's comments came with Mercedes widely tipped to be the team to beat while also fending off suggestions that they have used a loophole in the new regulations, which have allowed them to steal a march with the compression ratio in their power units.
McLaren, Alpine and Williams all use Mercedes engines while Red Bull have also been accused of exploiting the loophole.
The row prompted Hamilton, who won six of his seven titles during a 12-year spell with Mercedes before switching to Ferrari in 2025, to call for clarity.
"I hope we are in the mix (this season)," Hamilton told Sky Sports.
"Apart from Mercedes, we all look like we are quite close but we don't know what fuel loads everyone is on.
"There are whispers of certain fuel loads that Mercedes are on. There are whispers of extra power that they have that the rest of us don't have. And the compression ratio sort of thing."
Hamilton added he hoped the sport's governing body would resolve the matter.
"Hopefully that gets sorted and the FIA take care of that to make sure we are all starting on an equal playing field," he said.
Hamilton, 41, completed 52 laps in the morning session before handing over testing duties to Ferrari team-mate Charles Leclerc who clocked the third quickest time of the day.