Russell was second fastest in the day's opening practice session before going one better later in the day.
His performance was supported by team-mate Lewis Hamilton, who was fourth best on Friday just 0.2s back.
It marks a significant turnaround from the team which has struggled to extract the best from the W13 throughout the opening four races of the season.
“I don't really understand it, to be honest, why we hit the ground running,” Russell admitted at the end of Friday's running.
“I think we always knew ahead of this weekend that the conditions should suit us better, because we've been struggling a lot of the time to warm up, even in Bahrain when the track was really rough.
“But this is the first real hot race of the season, so for sure it has played a big factor.”
Conditions in Miami saw ambient temperatures of above 30 degrees, while track temps were over 50 as opening practice began.
That had cooled by some 10 degrees by Free Practice 2, yet Mercedes remained towards the top end of the timesheet throughout both sessions.
The squad has three development parts on the car this weekend, namely a new front wing end plate, new rear wing, and new beam wing.
However, Russell is unsure of their full impact as he was unable to debrief following Free Practice 1 after he was called to the stewards' room.
What was clear though was that porpoising was at least less of an issue, though was still apparent towards the end of the second practice session.
“We know that it will always be there,” Russell admitted.
“Maybe it's working at a lower range compared to where we were previously.
“Fur sure, in that high fuel run, I was feeling it and it didn't seem to be any better.
“Imola obviously wasn't a strong circuit for us but so far from what we can see the car was working well around Miami.”
F1 returns to the Miami International Autodrome for Free Practice 3 at 03:00 AEST on Sunday.