* Williams sisters beaten in second round
* Champion Serena loses to Spain's Muguruza
* Venus goes out to Schmiedlova (Recasts after Serena Williams defeat)
PARIS, May 28 (Reuters) - Defending champion and world No.1 Serena Williams suffered a crushing 6-2 6-2 defeat by Spain's Garbine Muguruza in the second round of the French Open on Wednesday to follow her elder sister Venus out of the second grand slam of the year.
On a chilly day in Paris, Serena never looked the force that has won 17 major titles. Her 20-year-old opponent broke the American's serve twice early in the first set and made the most of a series of unforced errors by Williams.
The 35th ranked Spaniard kept up the pressure and the match turned out to be a repeat of the nightmare of 2012, when Williams lost in the first round, rather than the glory of 2013 when she beat Maria Sharapova in the final here.
A netted return from Williams sent the Spaniard into ecstasy and even Williams's mother, Oracene, smiled as Muguruza leapt about the court. "Of course, it's amazing," Muguruza said. "I had to be really aggressive and focused.
"Today is a great day. I'm so happy," she shouted.
Serena said: "It was just one of those days, you can't be on (your game) every day. It's not the end of the world, there's always next year."
The Williams sisters had been due to meet in the third round but Venus had felt the chill of defeat when she became the first seed to lose in the second round, falling 2-6 6-3 6-4 to Anna Schmiedlova of Slovakia.
The 29th seed, wearing long sleeves to stay warm in temperatures of 14 Celsius, has never won the claycourt grand slam in 16 visits and the former world No.1, now 32, may have to accept she never will.
Schmiedlova, aged 19 and ranked 56 in the world, wasted two match points on Williams's serve before the American hit wide to set up a third, and she won the match with a fine backhand cross court.
"This is the biggest win of my life," said Schmiedlova, who broke down when expressing how much Venus had meant to her when she was growing up.
Venus had got off to a solid start against Schmiedlova, winning four games in a row to take the opening set.
Her opponent, however, stepped up a gear and after an early exchange of breaks, stole the nine-times grand slam champion's serve in the seventh game of the second set and levelled the contest when her opponent netted a forehand.
Schmiedlova pulled through thanks to a series of fine passing shots. Venus Williams was diagnosed with an auto-immune disease in September 2011 and since losing in the fourth round of Wimbledon that year, has lost in the first or second round of every grand slam she has taken part in.
Russian Mikhail Youzhny, the men's 15th seed, joined her in heading for the exit when he lost 6-0 6-3 3-6 6-4 to Czech Radek Stepanek.
Eugenie Bouchard, the 18th seed, recovered from a slow start in chilly conditions to beat Julia Goerges of Germany in three sets, winning eight in a row to set up her winning position.
Gilles Simon, the 29th seed in the men's draw, eased past Alejandro Gonzales of Colombia 6-4 6-0 6-2. (Reporting By Robert Woodward, editing by Alan Baldwin)