Barcelona - Formula One fired up its final four-day pre-season test on Tuesday with British teenager Lando Norris top of the time sheets for McLaren and Red Bull's Pierre Gasly close behind while Mercedes still lagged near the bottom.
Italian Antonio Giovinazzi had been fastest for Alfa Romeo at lunchtime with a fastest lap of one minute 18.589 seconds but Norris, 19, put in a 1:17.709 in the afternoon sunshine on the soft C4 tyres.
The McLaren stopped twice on track, however, the first time due to an hydraulics issue.
Fernando Alonso, Spain's two times world champion who left McLaren and Formula One at the end of last year, was there to watch.
Canadian Lance Stroll went third for Racing Point on the fastest C5 tyres, ahead of Ferrari's Sebastian Vettel on harder C3 rubber.