* Vettel goes quickest
* Paddock mourns De Villota
* Perez hits barriers
* Williams fined 60,000 euros for loose wheel (Adds quotes, Williams fine, tributes for De Villota)
By Alan Baldwin
SUZUKA, Japan, Oct 11 (Reuters) - Formula One world champion Sebastian Vettel was fastest on Friday in free practice for a Japanese Grand Prix that could secure his fourth successive title.
The German, who will wrap up the championship if he wins on Sunday and Ferrari's Fernando Alonso fails to finish in the top eight, lapped with a best time of one minute 33.852 seconds in an afternoon session with plenty of incidents.
"The car balance is decent, but I think we can still improve," said the Red Bull driver, who had been third and behind the two Mercedes drivers in a morning session led by Lewis Hamilton.
"It's good to see we are up there today, but it's not always so important; we need to step up our game for tomorrow, as Mercedes will be strong in qualifying."
Alonso, 77 points adrift of the German with five races remaining, ended the day with sixth and 10th places on the two timesheets.
"We were not as competitive as we wanted to be and now we must try and improve, putting together a series of changes that we already have in mind," he said.
As the Spaniard got out of the car and removed his helmet at the end of the second session, he was shocked to be told of the death in Spain of compatriot and former Marussia test driver Maria de Villota.
The news left the Suzuka paddock mourning a friend, with emotional tributes to a woman who had remained passionate about the sport despite losing an eye in a life-threatening test accident last year.
While Vettel took up his familiar place at the top of the timesheets, others hit trouble.
Lotus's Kimi Raikkonen, Pastor Maldonado for Williams and McLaren's Sergio Perez all went off in the afternoon.
Perez's crash was the hardest, with the Mexican slamming sideways into the barriers at the Spoon curve, while Raikkonen went into the gravel half an hour from the end of practice but was still fourth fastest.
"I was setting the car up for the entry to Spoon, put my right rear wheel onto the grass and just lost the back end," said Perez.
"The impact was quite hard but fortunately I'm fine."
First practice ended with the teams two by two on the timesheets - Hamilton and Nico Rosberg followed by the Red Bulls, Ferrari and Lotus pairings.
Hamilton's best time of one minute 34.157 seconds was set on the hard tyre on a gloriously sunny morning at the Honda-owned circuit overlooked by its giant Ferris wheel. The 2008 champion was only sixth in the afternoon.
Vettel, who is chasing his fifth win in a row and has been on pole position in the last three, had been 0.611 off the 2008 champion's pace in the morning but that was soon shown to be deceptive.
His Australian team mate Mark Webber, using a new chassis after retiring at last weekend's Korean Grand Prix with his Red Bull in flames after being hit by Force India's Adrian Sutil, was second fastest in 1:34,020.
Vettel has won three of the last four races at Suzuka, a favourite track where he won the 2011 title and can count on the support of a large contingent of local fans.
Ferrari's Brazilian Felipe Massa, runner-up at Suzuka last year, was faster than Alonso in both sessions while Australian Daniel Ricciardo, who replaces Webber at Red Bull in 2014, was in the top 10 throughout the day for Toro Rosso.
The main morning incidents came from those well down the pecking order, with Venezuelan Maldonado parking up at Spoon when his left rear wheel came off and bounced away. He then went into the barriers in the afternoon at Degner.
Stewards fined Williams 60,000 euros ($81,100) for the failure to fasten the wheel securely.
Marussia's French driver Jules Bianchi, who has a meaningless 10 place grid penalty for the race after collecting a third reprimand of the season in South Korea, and Caterham's Giedo van der Garde ended up in the tyre wall at Degner.
Bianchi did not take part in the second session with mechanics rebuilding the car. ($1 = 0.7395 euros) (Editing by Ed Osmond)