* Federer sees off local favourite Tomic
* Chardy upsets Del Potro to reach last 16
* Williams and Murray win in straight sets
By Nick Mulvenney
MELBOURNE, Jan 19 (Reuters) - Roger Federer was in no mood to let a trend develop after the first real upsets of the week at the Australian Open tennis on Saturday and slapped down local upstart Bernard Tomic with authority to reach the fourth round.
Juan Martin del Potro stunned Federer to win the U.S. Open in 2009 but the Argentine sixth seed was on the receiving end of the shock on Saturday when an inspired Jeremy Chardy led a four-strong French charge into the last 16.
Serena Williams and Andy Murray never looked like losing sometimes challenging contests earlier in the day but defending champion and world number one Victoria Azarenka had a closer call and was forced to dig deep for her victory.
All eyes were on the evening match in Rod Laver Arena, however, where Tomic had been talking up his chances of translating his good early-season form into a victory over a player rated by many as the best to ever pick up a racket.
Tomic gave his best and came within two points of winning a thrilling second-set tiebreak but the 17-times grand-slam champion simply upped the gears, pulled out a couple of extraordinary winners, and raced away to a 6-4 7-6 6-1 victory.
"I had to be able to bring the whole repertoire to the court today, defence and offence, which I enjoy," said the second seed, who next faces Milos Raonic, before offering some advice to Tomic.
"I think it's important to be confident but obviously you respect the game and you respect the other players. I think he has a lot of respect for me."
Del Potro battled back from two sets down to level his third-round 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. "It's a big win for me, maybe the best of my career."
After five days without any upsets of note at the year's first grand slam, two came along within minutes.
As Chardy was packing up his rackets on Hisense Arena, Italian Andreas Seppi was securing his place as the Frenchman's next opponent by wrapping up a 6-7 6-3 2-6 6-4 6-2 win over Croatian 12th seed Marin Cilic, a semi-finalist in 2010.
Azarenka had to come back from a break down in the deciding set to avoid the same fate against injury-hampered American Jamie Hampton and her relief at her 6-4 4-6 6-2 win was clear.
"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'."
Williams, seeking a sixth title at Melbourne Park, also wobbled a bit at 3-0 down in the second set against world number 72 Ayumi Morita after losing her serve for the first time in the tournament.
The third seed showed no discomfort from the ankle strain she sustained in the opening round, however, and 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."
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 a tricky 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 a set and a break down before he charged 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 nearly five hours he spent in the sweltering heat on Thursday in his second-round tie.
"We have a lot of good players," said Chardy. "I think everybody starts to play well this year. I don't know what is the thing. We just play good."
Japan has also had a good tournament but Kimiko Date-Krumm's fairytale run came to an end with a 6-2 7-6 defeat to Serbian Bojana Jovanovski, who was born two years after her 42-year-old opponent made her debut at Melbourne Park. (Editing by Clare Fallon)