Antonelli took part in the afternoon session on Thursday in Bahrain and clocked one minute 32.802 seconds in the final hour, pipping McLaren's Piastri, who was just 0.058 seconds slower.
They were the only two drivers to break the 1:33 barrier on the Sakhir circuit, the fastest times seen so far in 2026 testing.
Max Verstappen recorded an astonishing 139 laps in his Red Bull on the day and was third, clocking 1:33.62, followed by the Ferrari of seven-time world champion Lewis Hamilton (1:33.408) and title holder Lando Norris (1:33.453) in the other McLaren.
Mercedes have been looking good, as predicted, with George Russell having topped Wednesday's session.
Piastri was also second-fastest on Wednesday and has sounded positive about the new McLaren amid all the massive regulation changes, overhauled aerodynamics and revised power units for the 2026 campaign.
Earlier, Norris, who's looking forward to kicking off the defence of his title in Melbourne on March 8, had suggested there was still "quite a bit" to improve with his McLaren as they seek to battle with the other big three - Red Bull, Mercedes and Ferrari.
"At the minute, we're a little bit off, so to match the race pace of some of the others, we have to push a bit more and then we have more degradations," Norris told a press conference in Bahrain.
"I would love to win, but I don't think there's like a responsibility. Always in life I'll do my best to defend it and continue it, but it's a new season with loads of new challenges.
"It's not as simple as just continuing from last year and saying it's the same thing again. You know at the minute we have to improve the car quite a bit if we want to compete a bit more to be confident for the first race, but I feel confident.
"I feel better than I've ever done. Certainly after last year in winning the championship, that's given me the confidence, so it's reassuring to know that I've done it once and therefore I believe I can do it again.
"That's a good thing, but it's a long season and I'll make sure I do my part. Together as a team we'll make sure we give ourselves the best opportunity to do it again."
