This weekend's crucial Japanese Grand Prix looks likely to become a lottery, thanks to soaking rain that lashed the Suzuka circuit on Friday and is expected to continue on Saturday.
McLaren's Heikki Kovalainen emerged quickest in the first session held on a track wet from early-morning showers, while the afternoon practice, headed by German Adrian Sutil for Force India, was blighted by torrential rain.
So bad were the conditions that no flying laps were recorded for the first hour of the second 90-minute session.
Neither McLaren nor Force India were expected to be among the front-runners this weekend with the flowing Suzuka layout expected to favour Red Bull, who have excelled on circuits featuring similar high-speed corners this year.
Germany's Sebastian Vettel led Red Bull's charge on Friday, the 22-year-old finishing in second position in afternoon practice, six-tenths of a second behind compatriot Sutil.
Vettel is 25 points behind championship leader Jenson Button in the title race, and is still a mathematical -- if unlikely -- chance to win the world title with three races remaining.
Button, who is 15 points ahead of teammate Rubens Barrichello in the drivers' standings, elected to sit out the entire afternoon session after finishing a lowly 18th in the morning.
Barrichello and Kovalainen also elected to stay in their garages on a miserable afternoon, with most drivers only venturing out in the final 10 minutes.
Further rain is forecast for the third and final practice session at Suzuka on Saturday morning, although afternoon qualifying, and Sunday's race, are expected to be held in dry conditions.
The changing conditions turn the weekend into a guessing game for teams and drivers alike as they attempt to find the optimum set-up for the testing layout.
With the last two Japanese races being held at Fuji Speedway on the other side of the country, the teams and drivers have no relevant technical information to draw upon for Saturday's all-important qualifying session.
Suzuka last hosted a Grand Prix in 2006, when F1 cars sported grooved tyres and were shod by two different tyre manufacturers.
The final part of the circuit, from Turn 17 through to Turn 7, has been re-surfaced in the ensuing years, meaning tyre wear rates are different to those experienced three years ago.
Slick tyres returned to F1 this year for the first time since 1997.
Rain and the Japanese Grand Prix have gone hand-in-hand in recent years.
All of Saturday was washed out at the 2004 race at Suzuka, with Typhoon Ma-On causing both qualifying and the race to be held on Sunday.
Qualifying was a near washout in 2005, a race won by Kimi Raikkonen after the Finn charged through the field in his McLaren from a rain-affected 17th on the grid, while the 2007 event was held in horrendous conditions.
That race, won by Lewis Hamilton, is best remembered for Vettel running into the back of 2009 teammate Mark Webber while the field was under safety car conditions, costing both drivers a potential podium finish.