George Russell finished the Bahrain test on top of the timesheets for Mercedes, with Mick Schumacher sixth in the Alfa Romeo.
Russell set the pace with a 1:29.029, the quickest time of the two-day in-season test and one that was almost three seconds faster than his best effort in qualifying in an ailing Williams at the Bahrain GP.
The British rookie also went past the 100-lap mark in the W10.
Sergio Perez was only six hundredths of a second slower for Racing Point, who also struggled in last weekend's race.
But all eyes were once again on Schumacher on Wednesday, with the German taking on his second day of F1 Testing.
After making his F1 debut with Ferrari on Day One, Schumacher was in the Alfa Romeo to close off the test and completed 70 laps, finishing sixth and 0.969s off Russell's leading benchmark.
Schumacher's time, albeit on softer tyres, was quicker than Antonio Giovinazzi's in Bahrain qualifying.
Sebastian Vettel, replacing Schumacher in the Ferrari, was third and also racked up over a century of laps as the Scuderia enjoyed a reliable test after Charles Leclerc's engine issues in Sunday's race.
Carlos Sainz only completed 21 laps in the McLaren but finished fourth, while Toro Rosso's Daniil Kvyat was fifth.
Alex Albon, the London-born Thai and Kvyat's team-mate, was just behind Schumacher in seventh and topped the mileage count with 135 laps on the board.
Including Russell, there were four British drivers in action on Wednesday.
Dan Ticktum was the next highest placed in ninth, with the Red Bull protege making his F1 Testing debut. Lando Norris was 13th but only ran in the morning session, and finished just ahead of Renault youngster Jack Aitken who, as well as Ticktum, completed more than 100 laps.
Fernando Alonso was back in action, too, with the two-time world champion once again completing a Pirelli tyre test for McLaren. He finished 11th, splitting the two Haas drivers - Romain Grosjean and Pietro Fittipaldi.
Apart from Sainz, who split his day with McLaren and Pirelli tests, Williams' Nicholas Latifi propped up the timesheets with the team facing a three-second deficit to the leaders - similar to their early F1 2019 form.
An enthralling F1 2019 season resumes in China on April 12-14, live only on Sky Sports F1, while the next in-season test takes place following May's Spanish GP in Barcelona.
Sky Sports F1 is the home of live and exclusive F1. Find out more here to watch the 2019 season live