* Djokovic, Sharapova, Radwanska, Ferrer through
* Weather cooler after Thursday's heatwave (Writes through)
MELBOURNE, Jan 18 (Reuters) - A welcome wind blew away the stifling heat at Melbourne Park on Friday but there was no diverting Novak Djokovic or Maria Sharapova from their paths to the last 16 of the Australian Open on day five of the tournament.
Djokovic remained on course for his third straight title as he marched into the fourth round without giving up a set after successfully negotiating a tricky tie against Radek Stepanek with a 6-4 6-3 7-5 win.
Sharapova was even more impressive and, although denied a third successive 6-0 6-0 win of the week, looked every inch the title contender in her 6-1 6-3 demolition of seven-times grand slam champion Venus Williams.
Fourth seed Agnieszka Radwanska also continued her red-hot start to the year, notching up win number 12 with a 6-3 6-1 victory over Britain's Heather Watson in their third round tie.
Playing with the roof closed on Rod Laver Arena because of a few drops of rain that accompanied the cooler weather, Djokovic had to work for every point against Stepanek, who charged the net 67 times.
It was little more than a good workout for the world number one, however, and before launching a broadside at disgraced cyclist Lance Armstrong, he complimented the 34-year-old Czech on his contribution to the match.
"He loves the big stage," the Serbian said. "You saw how much fun he had. I also had a lot of fun playing. It was a very entertaining match. He's very skilful, comes to the net, never gives you the same ball twice."
Sharapova's flurry of fist pumps after she wound up her match against Williams showed how much the victory meant to the second seed, who has been in ruthless form this week despite missing her one warm-up tournament with a collar-bone injury.
"I think the reason I started so well in this tournament is because I knew that I had to," said the Russian 2008 champion.
"I knew I was coming in without any match play. It's tough. I hadn't played a professional match, a real match, in over two months."
The absence of the injured world number four Rafa Nadal has left a gaping hole in the top half of the draw and fourth seed David Ferrer eased further into it with a 6-4 6-2 6-3 win over another entertainer, 2006 finalist Marcos Baghdatis.
The Spanish baseliner showed more than a few decent touches of his own - most notably a sumptuous backhand lob - as he set up a fourth-round contest against Japan's Kei Nishikori.
Big-serving Tomas Berdych was a 6-3 6-2 6-2 winner over Jurgen Melzer and will face Kevin Anderson in the next round with a likely quarter-final meeting with Djokovic on the line.
"Kevin Anderson has a huge serve so it could be like playing against myself," the Czech fifth seed said of the South African, who upset 22nd seed Fernando Verdasco 4-6 6-3 4-6 7-6 6-2.
Djokovic must first get past Swiss Stanislas Wawrinka, who ended the 2013 challenge of the American men at Melbourne Park when he beat Sam Querrey 7-6 7-5 6-4.
Eighth seed Janko Tipsarevic outlasted Frenchman Julien Benneteau 3-6 6-4 2-6 6-4 6-3 in a second successive five-set match but Melbourne's changeable weather seemed to trouble him as much as the length of his contests.
"The weather in Australia, I have to say it's crazy," he said. "Today was beautiful, but it was really, really windy. Who knows what's going to happen tomorrow? Probably we'll have rain and we're going to end up playing indoors."
The Serbian will next face Spanish 10th seed Nicolas Almagro, who ended Jerzy Janowicz's maiden Australian Open with a 7-6 7-6 6-1 win in a match that was free of the histrionics that accompanied the Pole's second round victory.
Janowicz's compatriot Radwanska claims she is not superstitious, which might be a good thing as she will face 13th seed Ana Ivanovic in the fourth round as he looks to extend her winning streak to 13 matches.
"I just hope I keep going," last year's Wimbledon finallist said. "Of course now it is going to be harder and harder playing against seeded players."
Ivanovic won the battle of the Serbian former world number ones against Jelena Jankovic 7-5 6-3 and was later joined in the fourth round by another former French Open champion, Li Na.
China's Li, who beat Sorana Cirstea 6-4 6-1, reached the final in 2011 and will fancy her chances of going deep again this year, especially as Kim Clijsters, who beat her in both the last two years, has now retired again.
American teenager Madison Keys was knocked out by German fifth seed Angelique Kerber in the opening match on Rod Laver Arena.
"I'm taking away some positives. But right now, it kind of sucks," the 17-year-old said after her 6-2 7-5 defeat. (Editing by Justin Palmer)