* Nadal impresses against Monfils
* Murray, Federer, Azarenka, Sharapova progress
* Wozniacki, Raonic upset
MELBOURNE, Jan 18 (Reuters) - Rafa Nadal beamed with delight after laying down an emphatic marker with a brilliant display of all-court tennis to charge into the last 16 of the Australian Open on Saturday.
The Melbourne heatwave may have broken but arch-competitor Nadal was still bathed in sweat after his two-hour 6-1 6-2 6-3 demolition of the talented but error-prone Gael Monfils.
Roger Federer earlier boasted of his fitness, Maria Sharapova felt a bit rusty and Andy Murray berated himself but all three eased into the second week.
Victoria Azarenka remained on course for her third straight title but may find the next hurdle trickier after setting up a fourth round tie against Sloane Stephens, who she beat amid controversy in last year's semi-finals.
While there was relief all round at the cooler temperatures after four days of stifling heat, Milos Raonic and Caroline Wozniacki both felt the stinging burn of an upset.
Nadal never looked in any danger of an early departure, barring a brief injury scare when he jarred his foot early in the second set.
Such was his dominance after he suppressed the Frenchman's early charge, the Spaniard was at a loss to pick out an element of his game that pleased him most.
"I think I have to say that I played well from everywhere, no?," he said.
"Tonight I think I played a great match ... (but) just one very good day. That makes me feel confident, but I am in the fourth round. That's all."
The top seed will play Kei Nishikori for a place in the quarter-finals after the Japanese ended American interest in the men's draw with a 7-5 6-1 6-0 win over Donald Young.
Federer looked at his own imperious best as he dismantled Teymuraz Gabashvili 6-2 6-2 6-3 to reach the fourth round at a grand slam without giving up a set for the 27th time.
The Swiss is aware that tougher tests lie ahead, starting with Frenchman Jo-Wilfried Tsonga in the next round, but was delighted to have rid himself of his back problem.
"I'm just happy not waking up like an old man," said the 32-year-old, who failed to get to a grand slam final for the first time since 2002 last year.
"It was a tough year last year and now for the last few weeks, I'm feeling much better... I've done the hard work and that's perhaps why I'm playing well. "
Murray's back problems are more recent and he admitted to feeling a bit of stiffness as he finds his feet again following surgery and four months on the sidelines.
Spaniard Feliciano Lopez gave him a big test in the first set tiebreak but faded after conceding the second and the fourth seed claimed his 14th straight victory over a left-handed opponent 7-6 (7-2) 6-4 6-2.
"It's been a good start," said the Wimbledon champion.
"Today was a big step up for me. Feliciano is a top 30 player. He's a tricky opponent to play. So it was a good test for me and I did well."
Murray next meets Stephane Robert, who prevailed 6-0 7-6 (7-2) 6-4 over fellow lucky loser Martin Klizan - the first time a late replacement has reached the last 16 in Melbourne.
Azarenka humbled Yvonne Meusburger 6-1 6-0 in exactly an hour to set up another meeting with American Stephens, who she beat last year after taking a medical timeout as she struggled to close out their semi-final.
"I have great memories of last year. That's all I keep for me," the Belarusian said of the incident which led to her being accused of gamesmanship. "We left it all last year here, and that's what is important for me.
Sharapova continued to don her distinctive ice jacket at end changes despite the cooler weather as she beat France's Alize Cornet 6-1 7-6 (7-6).
On the comeback trail after a shoulder problems cut short her 2013 season, the third seed accepted that she had won without playing her best tennis.
"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," the 2008 Australian Open champion said.
"That will come. I'm not worried about that."
Her boyfriend Grigor Dimitrov took a small step out of her shadow when he reached the fourth round of a grand slam for the first time by edging 11th seed Raonic 6-3 3-6 6-4 7-6 (12-10).
The Bulgarian prevailed after an 156-minute Margaret Court Arena thriller rounded off by the tensest of tiebreaks which he clinched on his fifth match point.
The 22-year-old has clearly tired of the nickname "Baby Fed", bestowed upon him because of the similarity of his free-flowing style to that of Federer.
"What can I say?. We have debated that for quite some time and we have said it loud and clear that my name is Grigor."
Wozniacki was left ruing her failure to convert more than five of her 16 break points in her 4-6 7-5 6-3 defeat at the hands of rising Spanish talent Garbine Muguruza.
"You know, sometimes it happens, you just don't take your chances," the 10th-seeded Dane lamented. "I won the first set, was up in the second. I don't think I did anything particularly wrong." (Editing by John O'Brien and Pritha Sarkar)