Hamilton set a lap of one minute 25.801 seconds to edge out team-mate Jenson Button by 0.275 secs before a spell of heavy rain hit the Albert Park track.
BBC: Lewis Hamilton fastest in Friday Australian GP practice
And 20, 000 Grand Prix fans have been warned not to attend the Silverstone race track on Saturday, as rain left thousands stranded in traffic jams near the site.
David Lehr, the Churchill Downs track superintendent, has downgraded the main track to "sloppy" due to consistent rain falling Saturday morning and early afternoon.
Kahne set the qualifying speed record on the Vegas track last year, but rain wiped out qualifying this week.
It said the second track along the stretch, closed for nearly a week after heavy rain, was expected to reopen by 05:00 GMT on Saturday.
"We understand that because of the heavy rain there was a minor landslip that has brought the tree and foliage onto the track, " explained the Arriva Trains spokesperson.
The track dried quickly after early morning rain, and a fierce battle followed.
More than 150, 000 spectators packed Churchill Downs in Louisville, where rain had fallen much of the day and left the track sloppy.
Rain is pooling on the brick walkways at the entrance to the track and around the Kentucky Derby Museum.
In addition, Costa Rica will provide the U.S. with computer software used to track biodiversity in its rain forests.
Depending on Rita's track, New Orleans could receive several inches of rain, straining the city's fragile levees.
On the track, Hamilton had survived a frantic last two laps in a late shower of rain to apparently score a superb win.
Ms Varma inspected the track at 08:30 GMT on Sunday but found conditions to be unraceable after 11mm of rain fell on Saturday, leaving large areas of standing water.
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