Ferrari's Sebastian Vettel set the pace in a wet final practice session at the Chinese Grand Prix.
The German was 0.710 seconds quicker than Williams driver Valtteri Bottas as neither Mercedes driver set a lap time in order to conserve tyres.
Only 14 drivers set times as the number of wet-weather tyres is limited and teams were saving them for qualifying.
Force India's Sergio Perez was third, the leading times all set on intermediate tyres as the track dried.
The fastest driver on the 'extreme' tyre, which is used only when tracks are at their wettest, was Toro Rosso's Carlos Sainz in fourth place.
There were no major incidents, although a number of drivers ran a little wide as they explored the grip limits of the Shanghai track in the difficult conditions.
Forecasts predict showers for qualifying, which starts at 08:00 BST. Sunday's race is expected to be dry.
Mercedes driver Lewis Hamilton already knows heading into qualifying that he can start the race no higher than sixth even if he is fastest in qualifying, as a result of a grid penalty for a gearbox change before the end of its six-race mandated life.
If the track is damp in qualifying, a mixed-up grid is in prospect to some extent as teams such as Red Bull, Toro Rosso and McLaren will expect to be relatively more competitive than in the dry.
Mercedes - with the best car as well as the best engine - will remain favourites to set the pace but Vettel will have hopes that he can challenge Hamilton's team-mate Nico Rosberg for pole position.