* Williams thrashes last year's finalist Errani 6-0 6-1
* Holder Sharapova battles past Azarenka in three sets
* American faces Russian in Saturday's women's final (Adds details in final para)
PARIS, June 6 (Reuters) - Maria Sharapova had better come armed with a bit more than the 12 aces she served to reach the final when facing her nemesis Serena Williams in the French Open showpiece on Saturday.
For the second match running Russian defending champion Sharapova produced a wildly fluctuating display on Thursday as she beat fierce rival Victoria Azarenka 6-1 2-6 6-4.
Williams had been forced to wait as Sharapova and Azarenka finished their rain-interrupted two-hour scrap - then came out like a hungry tiger to maul hapless Italian Sara Errani 6-0 6-1.
On the day the WTA celebrated its 40th anniversary, top seed Williams made a mockery of Errani's world ranking of five as she underlined her unshakeable grip on the women's game.
Fifth seed Errani, a claycourt expert who was runner-up to Sharapova last year, pocketed only 16 points and was mercifully put out of her misery by the world No.1 after 46 minutes.
Williams won 40 of her 52 points with winners, incredible statistics, yet she seemed unimpressed.
"I didn't necessarily go out there feeling great," she said. "It was more like, let's just see what happens and try to do the best I could," she added with chronic under-statement.
Sharapova, second in the rankings, has not beaten 31-year-old Williams since 2004, a 12-match losing sequence.
"I'd be lying if it doesn't bother me, obviously," Sharapova said of her record against the American who is looking for her first singles crown at Roland Garros since her maiden French Open title in 2002.
"I don't think it would be a pretty competitive statement if I said it didn't. I would love to change that around. Obviously, whatever I did in the past hasn't worked, so I'll have to try to do something different and hopefully it will."
Quite what that will be is anyone's guess as Williams, apart from a brief hiccup against Svetlana Kuznetsova in the quarter-final when she dropped her only set of the tournament, has looked unstoppable on the Parisian clay.
Errani never stood a chance.
The 26-year-old looked close to tears at 0-3 in the second set but avoided the dreaded "double bagel" by winning the fourth game of the second set when her opponent fired a backhand wide.
She raised her hands in celebration but Williams soon resumed the slaughter to win the most one-sided women's singles semi-final here since seven-times champion Chris Evert beat Camille Benjamin 6-0 6-0 in 1984.
Errani's post-match news conference was also a brief affair, the Italian using the word "unbelievable" several times to describe Williams' performance.
She also offered some hope for Sharapova.
"You have to have one of your best days and try to think she can have one bad day, like that you can win," Errani said.
Sharapova can hope, but she will have to play a lot better than against third-seeded Azarenka in a match that was intriguing and noisy but lacked sustained quality.
The Russian feebly dropped the opening game on serve to love with two double faults but responded to allow Azarenka only seven more points in the set.
Azarenka, playing in her first French Open semi-final, never usually ducks a scrap and began to return some of Sharapova's lusty blows with interest.
From 2-2 and with storm clouds gathering above, the Belarussian won the next four games, clinching the second set on a Sharapova double fault, one of 11 gifts her opponent served, before heavy rain sent the players scuttling for cover.
After a 30-minute break in play Sharapova secured the first break of serve in the decider to lead 2-1 but failed to capitalise as the following game turned into a "lucky dip" with Sharapova mixing two aces with four double faults.
The rollercoaster ride continued as Sharapova surged 5-2 ahead only for Azarenka to save four match points with some courageous shots that peppered Sharapova's baseline.
Two more double faults gifted Azarenka the game and when she held to love to make it 5-4 the tension was mounting.
As cool as you like, Sharapova then stepped up to the line, fired down two powerful first serves on her way to a 40-0 lead and finished off Azarenka with a skidding ace.
Despite the tenacity of the contest, the one-dimensional nature of the match was a poor advert for women's tennis, with the number two and three players in the world committing 61 errors between them.
Not that it bothered Sharapova who is through to her eighth major final.
"I think it doesn't just take big ball striking to get to a final of a grand slam. It takes a lot more than that," she said.
It will certainly take a lot more than that to halt Williams in her tracks on Saturday.
After the erratic nature of the women's semis, attention turns to Friday's mouthwatering last-four clash between top seed Novak Djokovic and seven-times French Open champion Rafa Nadal in a rematch of last year's men's final won by the Spaniard.
The winner will play either Nadal's compatriot David Ferrer or local favourite Jo-Wilfried Tsonga in Sunday's final.
(Reporting by Martyn Herman; Editing by Ken Ferris)