By Pritha Sarkar

PARIS (Reuters) - For a split second Garbine Muguruza stood wide-eyed and open mouthed as she seemed to have no clue if her lob had landed in -- but as she turned towards the VIP Box, the wild cheers proved she had dethroned Serena Williams to win the French Open title.

The 22-year-old Spaniard, who produced a pristine performance for an hour and 43 minutes on Saturday to deny Williams a 22nd major trophy with a 7-5 6-4 victory, was soon covered head-to-toe in clay after collapsing on to her back to celebrate her maiden grand slam triumph.

Her shirt, arms, white sweatbands, cheeks and hair were all caked in red dirt but Muguruza did not care a jot as she became the first Spanish woman to hoist the Suzanne Lenglen Cup since Arantxa Sanchez-Vicario in 1998.

"It (match point) was really weird. Serena was in front of the ball so I didn't know if it was in or out," said a beaming Muguruza, who was presented with the trophy by American great Billie Jean King.

"I looked at the umpire and he doesn't want to say anything. I was like, 'did I win Roland Garros?'

"When he said, 'game, set, and match'...it was like ...no way. I won. It was amazing."

Incredibly, the Caracas-born player had never claimed a claycourt title before winning what she called "the perfect final" on Saturday.

If she was the superstitious kind, she would have chosen not to flick through the record books -- as the last time a woman contested her first claycourt final in Paris, she got walloped 6-0 6-0 by an incumbent world number one.

Not only did Muguruza avoid suffering the same fate as Natasha Zvereva, who was humiliated by Steffi Graf in 1988, she did not even allow the current number one to win a set on Saturday.

'SPAIN IS LUCKY TO HAVE HER'

"She's world number two after this, only one more step to be number one... she's going to win many more grand slams," 1994 Wimbledon champion Conchita Martinez said of the new title-holder.

"She's a great player on any surface. Spain is very lucky to have this unbelievable tennis player."

Muguruza has a history of leaving Williams feeling ill at ease in Paris as the Spaniard is the only woman to have beaten the American there since 2012.

That 2-0 record includes a 6-2 6-2 second round win two years ago -- the heaviest defeat of Williams' 339-match grand slam career.

On the eve of Saturday's final, Williams said she had "learned a ton" from that defeat as it had propelled her "to many, many, many wins after that".

While it did help her to win 30 of her next 31 major matches to complete a second "Serena Slam" at last year's Wimbledon, where she beat Muguruza in the final, the lessons were not enough to carry her over the finishing line on Saturday.

"So many holes (in my game). I could have served better. I made a lot of errors," said Williams, whose bid to draw level with Graf's professional era record of 22 grand slam titles has hit the buffers since last July.

"I didn't play the game I needed to win. Garbine played unbelievable."

After one of the wettest weeks on record in Paris, Williams was forced to play a fourth match in as many days.

She did not look as listless as she had 24 hours earlier, but when she surrendered her serve to Muguruza in the fifth game by firing three backhand errors and a double fault, the signs looked ominous.

A match dominated by a barrage of heavy hitting from the baseline was not one for the purists but it was clear from the start that Muguruza planned to take Williams on at her own game.

A thundering backhand down the line left Williams chasing shadows as Muguruza bagged the first set and even when the Spaniard missed four match points on Williams' serve in the ninth game of the second set, it did not fluster her.

Only a few minutes later she produced the lob that had the umpire -- albeit after a few seconds hesitation -- announcing "jeu, set et match Mademoiselle Muguruza".

(Additional reporting by Julien Pretot, editing by John Stonestreet)