Mercedes' Lewis Hamilton posted the fastest time in an extremely wet final practice session at the United States Grand Prix.
Heavy rains have pounded the Circuit of the Americas for two days. Officials canceled Friday afternoon's practice and allowed the drivers to use only full wet tires Saturday morning.
With cars sliding and spinning through turns, Hamilton was well back before running through a lap of 1 minute, 59.517 seconds to take the top spot from Ferrari's Sebastian Vettel.
More heavy rain was expected, raising questions about afternoon qualifying.
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Race officials didn't immediately announce plans for qualifying. If officials decided to use practice times, Hamilton would earn pole position for Sunday's race as he seeks to win his second consecutive world title and third overall.
Sahara Force India's Nico Hulkenberg would start second because of a grid penalty given to Ferrari for an engine change.