By Alan Baldwin
AUSTIN, Texas (Reuters) - Final practice for the U.S. Formula One Grand Prix got under way behind closed doors on Saturday despite continuing rain and local flood warnings that forced promoters to deny access to spectators.
With Friday's second practice entirely washed out, and storms likely to threaten qualifying later, the final session could determine the starting grid for Sunday's potentially title-deciding race.
Drivers wasted no time getting on track, despite the conditions, and lapping in heavy spray. Several aquaplaned off, with Nico Rosberg breaking the front wing on his Mercedes as he went off.
The Circuit of the Americas had already told fans the gates would stay closed until after the session.
"For the safety of our fans and employees, today's opening has been delayed until noon," they said on Twitter before practice started at 10:00 a.m. CDT (1500 GMT), with drivers wasting no time in getting on track.
"We are continuing to prepare the grounds to safely accommodate our guests. FIA has elected to run FP3 (third practice), however we will open at noon," they added later.
The National Weather Service on Friday placed most of the state of 27 million people under a flood watch, warning of torrential downpours through the weekend.
If qualifying has to be canceled on Saturday, it could still be held on Sunday or final practice positions used to determine the grid.
The race is scheduled for 2 p.m., when the weather is expected to be better.
Britain's Lewis Hamilton can win his third world championship in Austin if he scores two points more than team mate Rosberg and nine more than Ferrari's Sebastian Vettel.
There are three more races remaining after Austin, with Mexico -- hit by Hurricane Patricia on Friday -- the following weekend.
"My main worry is for my country," Mexican driver Sergio Perez said after Friday's second practice was canceled due to the rain. "Mexico is being hit by the largest hurricane in its history and all my thoughts are with my people. I am praying for everyone to be safe."
(Editing by Frank Pingue)