Piastri leads McLaren 1-2 in Bahrain final practice
The Australian, who topped Friday's second practice, posted a best time of 1min 31.646, with his teammate Lando Norris second, six-tenths of a second slower.
This underlined the overwhelming feeling that pole position for Sunday's fourth round of the season is theirs to lose.
Max Verstappen, who moved to within one point of Norris in the drivers' standings with his win in Japan last weekend, looks to have his work cut out to make the front row after his Red Bull only finished in eighth, well over a second behind Piastri.
Ferrari had arrived in Bahrain with their bags full of upgrades after a disappointing start to the season - Lewis Hamilton's Chinese sprint win from pole aside.
And Hamilton led from Charles Leclerc in the early stages before Norris went clear by over a second.
The championship leader's lap was soon bettered by his teammate Piastri, the McLaren men well clear of the rest of the field.
Leclerc eventually finished third, eight-tenths of a second slower, ahead of the two Mercedes men George Russell and Kimi Antonelli, Alpine's Pierre Gasly, and impressive RB rookie Isack Hadjar.
Hamilton was 10th, Liam Lawson, back at RB, took 13th with the man who replaced him at Red Bull last weekend Yuki Tsunoda, plum last.
Grip was at a premium in the session given the sun beating down on a surface that has not been relaid since the circuit was built in the early 2000s.
"I'd probably say that's the least amount of grip I've had in an F1 car" Russell told his team engineer.
"Yeah, that's terrible," grumbled Verstappen after going off the track in the second sector to post a time three seconds slower than Piastri in second practice on Friday.
He went back to the pits to regroup as a virtual safety car was required when Nico Hulkenberg's Sauber came to a standstill ending the German's session.
With practice held in the seering desert sunshine, it may not be particularly meaningful for a race that will be run after sunset under floodlights and in cooler conditions but the pace of the McLarens will be a warning in any conditions.