Mercedes team boss Toto Wolff made headlines on Wednesday when he said Red Bull were the benchmark, and Russell continued that line by suggesting the team of Max Verstappen and Isack Hadjar had a "pretty scary" advantage.
Red Bull have suggested it was more likely to be the other way around as Formula One heads into a new engine era with major chassis changes as well.
"Just wait until Melbourne and see how much power they suddenly find. I already know that right now," four-times world champion Verstappen told reporters on Thursday.
Red Bull technical director Pierre Wache said Ferrari, Mercedes and McLaren appeared to be ahead. Ferrari's Charles Leclerc said both Mercedes and Red Bull looked strong.
"I think Red Bull showed a little bit more than Mercedes and they've been very impressive. I think Mercedes is hiding a massive amount, and only time will tell how much they’ve been hiding," said the Monegasque.
Russell was in the car for the first half of Friday's session, and handed over to Antonelli who set a best lap of one minute 33.669 seconds. Russell's time was 1:33.918.
Ferrari's Lewis Hamilton was third fastest, completing a hefty 150 laps and stopping on track at the end of the session in what may have been a deliberate running out of fuel.
McLaren's Oscar Piastri completed 161 laps, nearly three race distances of the Sakhir circuit, and was fourth fastest with Verstappen fifth and Hadjar sixth.
Hadjar sounded a more optimistic note than Verstappen, saying the new Red Bull engine seemed reliable and powerful.
"If anything it's going a bit better than we hoped for, more on the PU (power unit) side," he said.
Aston Martin's Lance Stroll was an eye-catching 4.4 seconds off the pace and faster only than Valtteri Bottas, who completed a mere 37 laps after an earlier stoppage, for newcomers Cadillac.
Teammate Sergio Perez managed 42 laps in the second half of the day.
Aston Martin managed to do 206 laps across three days, less than half the 422 of champions McLaren and 423 for Ferrari.
The second Bahrain test runs from February 18-20 with the season starting in Australia on March 8.
