* Sharapova sends warning to Serena
* To play Flipkens in fourth round
* Says tennis is clean of doping (Writes through with quotes)
By Ian Ransom
MELBOURNE, Jan 18 (Reuters) - A merciless Maria Sharapova continued her serene progress through the Australian Open draw on Friday, obliterating third-round opponent Venus Williams 6-1 6-3 and throwing down the gauntlet to her younger sister Serena.
The second seeded Russian cannot meet the tournament favourite until the final, but the 79-minute slaughter filled with blood-curdling shrieks is sure to provide food for thought for the 15-times grand slam champion.
With her two prior opponents left scoreless and humbled, Sharapova appeared set for a third successive 'double bagel' when she scorched to a 4-0 lead.
Mercifully, for herself and a stunned centre court crowd, 25th-seed Williams held serve to stave off another humiliation.
"I was a really determined player out there because I knew the tennis that she's capable of producing and playing," Sharapova told reporters of the seven-times grand slam champion.
"Despite what she's ranked or seeded, it doesn't matter when you go out on the court. She's been there. She's experienced enough to know no matter if you're playing the third round, the quarters, or the final, you have to be ready.
"I certainly was."
A much-hyped match-up was reduced to a one-sided rout, as Sharapova burned to a 5-1 lead in the second set.
Williams sparked raucous cheers by breaking Sharapova's serve and lifted the roof by holding her own to claw back to 5-3.
But Sharapova blasted an ace to seal the match, celebrating the victory with a fist-pumping scream.
The crushing loss amplified Williams's fall from her halcyon days, and left her stony-faced at her post-match media conference.
"Definitely not my best day today," said Williams, who has battled Sjogren's Syndrome, which causes fatigue and joint pain.
Belgium's Kirsten Flipkens, who has had compatriot and retired four-times grand slam champion Kim Clijsters helping her behind the scenes, is next to be thrown to the wolves against Sharapova.
Lance Armstrong's confession of doping in a television interview with Oprah Winfrey has caused a ripple among players at the Australian Open and Sharapova weighed in to say it was "sad" for the sport of cycling.
"I'm happy that our sport is as clean as it can be and that we're constantly tested. You know, we give whereabouts of where we are every single day of the year.
"Hopefully not on birthdays and Christmas Eve, that would be pretty tough.
"Although they did show up on my birthday and I was very disappointed. They did a couple of years ago. I said, 'unless you bring flowers, I'm okay with it'.
"But they came empty-handed."
(Editing by Greg Stutchbury)