Lewis Hamilton pipped team-mate Nico Rosberg to fastest time in final practice before qualifying at the Hungarian Grand Prix.
The world champion was just 0.098 seconds quicker than the second Mercedes on what was a less-than-perfect lap as a result of a mistake at the first corner.
Rosberg, who headed Ferrari's Sebastian Vettel and Red Bull's Daniil Kvyat, stopped in the pit lane at the end of the session.
Mercedes tech boss Paddy Lowe said the engine stalled but he did not know why.
Lowe added that he did not know who Mercedes' closest rivals would be, pointing out that after Red Bull had looked strong on Friday they had dropped back on Saturday morning.
Kvyat was 0.329secs behind Vettel and 1.218secs adrift of Hamilton.
Vettel's team-mate Kimi Raikkonen did not get a low-fuel run on the softer tyre because of a water leak and was 16th fastest.
It has been a difficult weekend for Ferrari so far, with Vettel unhappy with the car and having two spins on Friday and a front wing failure on Raikkonen's car on Friday before the engine problem on Saturday.
BBC F1 co-commentator David Coulthard said: "We have to question Ferrari's development - are they going down the right route? They had that win in Malaysia but it has stalled since then."
Toro Rosso's Carlos Sainz was an impressive fifth, ahead of Force India's Nico Hulkenberg and the Spaniard's team-mate Max Verstappen.
Fernando Alonso was eighth fastest, continuing McLaren's encouraging form continued on a track that does not expose the lack of power from the Honda engine as badly as other.
Team-mate Jenson Button, who has expressed his hopes of a top-10 grid position, was 0.383secs adrift and 13th fastest, having complained early in the session of a lack of rear grip.
Williams were surprisingly far down, with their faster driver Felipe Massa only 12th fastest and team-mate Valtteri Bottas 14th.
Qualifying, which starts at 1300 BST with BBC One coverage from 1210, takes place against the backdrop of the death of former Marussia driver Jules Bianchi last week from injuries suffered in a crash during last year's Japanese Grand Prix.