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Maria Sharapova is looking to regain her top form at the Aussie Open. Reuters

Maria Sharapova continued her erratic Australian Open campaign with a scratchy 6-1 7-6 (8-6) win over Alize Cornet that put her into the last 16 on Saturday but also sent her straight to the practice court in search of some sorely needed form.

Still rebuilding from a long layoff due to her troublesome right shoulder, Sharapova ran hot then cold against the 25th seeded Frenchwoman, her serve going awry in the hard-fought second set and only a late rally in the tiebreak saving the Russian from another potential marathon.

In her previous round, Sharapova had battled through the longest match of her career, a three-hour, 28-minute slug-fest against unseeded Italian Karin Knapp, played out on a 42 degrees Celsius (108 Fahrenheit) day.

Fatigue might have been a tempting excuse, but the Russian could hardly have used it, having felt fit enough to head back out on the court after a quick rest to iron out some worrying creases in her game.

"I just didn't have a great rhythm," Sharapova, seeded third, told reporters after bludgeoning 35 unforced errors and landing only half of her first serves.

"It's sometimes nice just to be able to come off the court and groove, just to get a good hit on the ball.

"I still feel like in certain situations I am a bit rusty and I'm not closing it out when I have to or maybe going for a little much or overthinking it a bit.

"That will come. I'm not worried about that. Those are the - as long as I feel like I'm doing the right things and I'm playing the way I want to play, if I'm making those types of errors, they are going to go in eventually."

SERVE ISSUES

Sharapova's win over Cornet was her seventh match since pulling the plug on her 2013 season in August, but the 26-year-old has made hard work of her opening three matches at Melbourne Park, where she won the 2008 title.

The loss of her once-potent serve since major surgery on her shoulder six years ago has long been her Achilles heel, and if her serve against Cornet was any indication, the old injury may still be weighing heavily.

Broken three times in the second set, the average pace of her first serves was 153 kph (95 mph) for the match, well down from her opening two rounds. Her second serve was also markedly slower.

Her decade-long tormentor Serena Williams, by contrast, has only been broken once and has been serving at an average just under 170 kph for the tournament.

Sharapova next plays Slovak 20th seed Dominika Cibulkova and has been spared a tougher run by a generous draw that offers eighth seed Jelena Jankovic as the highest seeded player before the semi-finals, where double defending champion Victoria Azarenka looms as the most likely opponent.

The Russian held out little hope of being completely free of the shoulder problems that have limited her to only one grand slam title win to the three prior to her major surgery.

"As I said, I think recovery for the rest of my career is going to be extremely important, making sure I do the right amount of work to the right amount of rest that I give it," she said. "But it's feeling good."

(By Ian Ranson; Additional reporting by Greg Stutchbury in Wellington; Editing by John O'Brien)