MELBOURNE (AFP) - Venus Williams slumped to a shock loss in the Australian Open second round Thursday while sister Serena stumbled but managed to keep her quest for a fourth title alive. Sixth seed Venus' exit at the hands of Spain's Carla Suarez Navarro was the biggest upset so far at the season-opening Grand Slam, overshadowing Serena's woes and a comfortable win for Russian fourth seed Elena Demenieva. It blows open a women's draw already weakened by the injury-enforced absence of defending champion Maria Sharapova. The seven-time Grand Slam champion went down to 20-year-old Suarez Navarro 2-6, 6-3, 7-5 in a two hour 14 minute marathon after squandering match point in the third set. She has now failed to win an Australian Open in 10 attempts but rejected talk she was jinxed at the tournament, preferring instead to lick her wounds and concentrate on her doubles camapign at Melbourne Park. "Losing's never the best fun, to say the least, but fortunately I've also had a lot of nice wins, too, so hopefully that will balance it out," the 28-year-old said. Her departure may strengthen sister Serena's chances, as a Williams sisters semi-final was on the cards if the siblings both progressed. However, Serena admitted she needed to lift her game if she wanted a fourth Australian title for her bulging trophy cabinet. The American second seed made heavy work of a 6-3, 7-5 win over Argentinian Gisela Dulko, declaring there was "lots and lots and lots of room for improvement". "Today I was at like a D minus at best, that's pretty far away" she said. "But it's good that I was able to win when I wasn't playing my best. I definitely will try to do better." Like Venus, she dominated her opponent early before losing focus. Unlike her sister, she was able to regroup for a win, saying the unexpected test would increase her match toughness as the tournament progressed. Meanwhile, Dementieva continued to quietly put together a strong run of form which could make her the dark horse in an unpredictable draw. The in-form Russian crushed the Czech Republic's Iveta Benesova 6-4, 6-1 to advance into the last 32 of the season-opening Grand Slam. The 27-year-old has followed up the Olympic gold medal she won at last year's Beijing Olympics with a formidible string of victories that has already delivered two titles in 2009, at Auckland and Sydney. But she played down talk of winning her first Grand Slam, preferring to remain low profile and concentrate on her tennis. "I don't think about all the pressure, the media attention, I'm here to play. That's what I like to do," she said. Another contender lurking in the draw is 2006 champion Amelie Mauresmo of France, seeded 20th this year but planning to draw on more than a decade of Grand Slam experience in her bid for a second Australian title. The 29-year-old came from behind against British qualifier Elena Baltacha for a 4-6, 6-3, 6-2 win. "I've been playing a lot of Grand Slams now, and I know that you cannot really define your game from what you can see in the first couple rounds," she said. "I'm expecting to play better and better as the tournament goes on. I'm definitely expecting a tougher match, tougher opponent next round. So I will have to step up and play some better tennis."