MELBOURNE (Reuters) - Juan Martin del Potro suffered the first real upset and Victoria Azarenka had a big scare, but Serena Williams and Andy Murray eased through challenging ties to reach the last 16 of the Australian Open on Saturday.

Juan Martin del Potro of Argentina hits a return to Jeremy Chardy of France during their men's singles match at the Australian Open tennis tournament in Melbourne January 19, 2013. REUTERS/Toby Melville

After five days at the year's first grand slam without any major shocks, two came along within minutes as Jeremy Chardy downed Argentine sixth seed Del Potro and Andreas Seppi showed 12th seed Marin Cilic the door at Melbourne Park.

Del Potro, the 2009 U.S. Open champion, had battled back from two sets down to level the contest but the mercurial Chardy grabbed a break in the decider and held his nerve to serve out for a 6-3 6-3 6-7 3-6 6-3 win.

"I had nothing to lose today so it was easy to play," said the world number 36. "I knew that if I wanted to win the match, I had to go for it from the start.

"It's a big win for me, maybe the best of my career."

As Chardy was packing away his rackets on Hisense Arena, Italian Seppi was securing his place as the Frenchman's opponent by wrapping up a 6-7 6-3 2-6 6-4 6-2 win over Croatian Cilic, a semi-finalist in 2010.

While Williams and Murray were troubled by their opponents but never looked like losing, defending champion and world number one Azarenka was forced to dig deep for her spot in the fourth round.

The Belarusian had to come back from a break down in the deciding set to overcome injury-hampered American Jamie Hampton 6-4 4-6 6-2 and her frustration and relief were palpable.

"She took a medical timeout but she rips winners all over the place," said Azarenka. "I was like 'can I have a back problem? I'm feeling great but I'm missing every shot'.

"Winning ugly always means that you overcome something," she added.

NONSENSE TENNIS

Williams, seeking a sixth crown at Melbourne Park and a third successive grand slam title, was 3-0 down in the second set against world number 72 Ayumi Morita after suffering a service break for the first time in the tournament.

Showing no discomfort from the ankle strain she sustained in the opening round, the third seed stormed back to win the next six games and dismiss the Japanese 6-1 6-3.

"I feel good," the 31-year-old American said. "I feel today was actually a really good match for me. I was involved in a lot of longer points, something I definitely wanted. I hope I can keep this level up and go higher."

U.S. Open champion Murray berated himself for playing "nonsense" tennis at times but eventually broke down his Lithuanian practice partner Ricardas Berankis 6-3 6-4 7-5 after 132 minutes in the Melbourne sun.

"Sometimes when you are struggling, you get very frustrated," said the British third seed. "I need to strike the ball better. My timing was off and I was leaving a lot of balls very short and allowing him to dictate some of the points."

Murray will next face either Gilles Simon or Gael Monfils, who meet later on Hisense Arena in a match that guarantees four Frenchman will be in the fourth round for the first time since 1998.

Ninth seed Richard Gasquet's progress was by no means smooth and he was in a hole of his own at a set and a break down before charging back to beat Croatian Ivan Dodig 4-6 6-3 7-6 6-0.

His seventh seeded compatriot Jo-Wilfried Tsonga had an easier day, hammering Blaz Kavcic 6-2 6-1 6-4 as the Slovenian paid the price for the four hours and 50 minutes he spent in the 40 degree Celsius (104 Fahrenheit) heat on Thursday in his second round tie.

"We have a lot of good players," said Chardy of the French charge. "I think everybody start to play well this year. So I don't know. I don't know what is the thing. We just play good."

Australia will be hoping that Bernard Tomic can maintain the upset trend in the evening match on Rod Laver Arena when he comes face to face with Roger Federer.

(Editing by Patrick Johnston)

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