By Miles Evans

MELBOURNE (Reuters) - When rare Melbourne drizzle put a dampener on the start of day five, it was left to former champion Serena Williams to roll back the years and ignite the Australian Open on Friday.

Unseeded this year after a string of injuries dogged her 2006 season, the two-times former champion looked dead and buried against fifth seed Nadia Petrova at a set and 5-4 down with the Russian serving for a place in round four.

But Serena dug deep and shattered the Russian's resolve with a 1-6 7-5 6-3 victory and a place in the last 16 against red-hot Serb Jelena Jankovic.

Though delighted with her comeback victory -- her third in the tournament against the luckless Muscovite -- Williams seemed more concerned her father was watching her efforts.

She was seen mouthing 'Hi Dad, call me' to the cameras as she celebrated her win and revealed to reporters after that it was a plea to her mentor Richard to get in touch.

"I'm trying to reach my dad, I haven't been able to reach him for a few days. I know he's watching so maybe he'll call me. Whenever I call him my number doesn't come up. It's so frustrating."

Defending champion Amelie Mauresmo happily strolled into a last 16 place against unseeded Czech Lucie Safarova after a 6-3 6-1 win over world number 83 Eva Birnerova of the Czech Republic.

Third seed Svetlana Kuznetsova bustled her way through at the expense of fellow Russian and 26th seed Maria Kirilenko 6-1 6-4.

In the early men's action, Serb prodigy Novak Djokovic dropped his first set of the week when coming through in four sets against Thailand's Danai Udomchoke, the last Asian representative in the men's draw.

Next up for Djokovic, subject to the Swiss maestro beating Mikhail Youzhny, is a dream fourth-round match-up with Roger Federer, and the 19-year-old has anything but surrender on his mind.

"Federer is a big challenge," he said after his 6-3 6-4 5-7 6-1 victory.

"Everyone knows his game but he's so perfect and makes everything look so easy. But if I go on court with a white flag then what am I doing here?"

While local gardeners and farmers danced with delight as the rain fell on drought-blighted Victoria, organisers cursed as a four-hour rain delay hit the uncovered outside courts.

When the rain stopped Jankovic wasted little time in confirming her place against Williams.

The 11th seed, already a winner this year in Auckland, came through against Victoria Azarenka of Belarus 6-3 6-4 in 93 minutes to reach the fourth round for the first time.

And in a week that's seen drunken brawls among fans and criticism of the tournament's extreme heat policy, the unseasonal rain and a reported indecent assault on a five-year-old boy on Monday at Melbourne Park added to organisers' headaches.

"We have stepped up security. More people are patrolling the areas and have been asked to be extra vigilant," Tennis Australia CEO Steve Wood said, adding that security would be beefed up for the remainder of the tournament.