* Nadal celebrates birthday with a win
* Djokovic drops a set for the first time
* Sharapova and Azarenka advance
PARIS, June 3 (Reuters) - All the complaints about lousy weather, windy courts and rotten schedules were forgotten as the sun finally shone on birthday boy Rafa Nadal's pursuit of a record eighth French Open title on Monday.
Nadal spent the first week labelling Paris "too cold", the Chatrier Court "too windy" and the playing schedule "a joke" after three wobbly performances but he was all smiles on day nine of the claycourt major as the mercury finally crept over 20 degrees Celsius for the first time in this year's event.
The Spaniard, bidding to become the first player to win the same major eight times, took a step closer to a possible semi-final blockbuster against Novak Djokovic with a 6-4 6-1 6-3 last-16 win over Japan's Kei Nishikori.
After the fans serenaded Nadal by singing 'Happy Birthday', the 27-year-old smiled sheepishly as organisers wheeled out a giant, four-tier cake decorated with rackets, golden tennis balls and an array of sparklers.
Nadal's grin would have grown wider after watching his next opponent, Stanislas Wawrinka, dragged into a four-hour struggle before the Swiss squeezed past French favourite Richard Gasquet 6-7(5) 4-6 6-4 7-5 8-6.
Just how Wawrinka's weary body will recover for the task ahead is anyone's guess but for now Wawrinka will be glad to have survived a lively marathon in which he raged against the umpire, ordering him to "change the guy...making the bad linecalls", and produced 92 rip-roaring winners.
Philipp Kohlschreiber earned the distinction of becoming the first player to snatch a set off Djokovic at the tournament but that was as good as it got for the German.
Top seed Djokovic, who left Roland Garros in tears two days ago after learning of the death of his first coach Jelena Gencic, kept his emotions in check for a 4-6 6-3 6-4 6-4 win.
Describing Gencic as his "second mother", Djokovic said her death had strengthened his resolve to win the only grand slam title missing from his collection.
"I feel in her honour that I need to go all the way. It gives me that inner strength to push even harder," he said.
Women's champion Maria Sharapova knows how it feels to lift the French Open cup and she put American upstart Sloane Stevens in her place with a 6-4 6-3 defeat. She will next run into Serbia's Jelena Jankovic.
There were no trophies on offer for reaching the second Monday of the French Open but if there were, Victoria Azarenka would have been declared the 'best wailer', Tommy Haas the 'best golden oldie' and Mikhail Youzhny the 'best racket basher'.
Australian Open champion Azarenka's clash with 2010 Roland Garros winner Francesca Schiavone had been billed as the pick of the fourth-round games but it failed to draw the crowds.
The discerning Parisian fans chose to turn a deaf ear to the ear-splitting sounds emitted by both women on Philippe Chatrier Court first thing in the morning as third seed Azarenka won 6-3 6-0 to set up a last-eight date with Russian Maria Kirilenko.
The Belarussian's sound effects have been compared to those made by jet liners or even women giving birth and with Schiavone also chipping in with her kung-fu style yelps, it was definitely not a contest for the purists.
It was only when a victorious Azarenka twirled around to blow kisses to the four corners of the court that fans began to flood in - ready for Djokovic's game with Kohlschreiber.
A short hop away on Suzanne Lenglen Court, another German, Haas, proved that there was plenty of life left in his 35-year-old limbs as he raced past a tormented Youzhny 6-1 6-1 6-3 to become the oldest Roland Garros quarter-finalist since Istvan Gulyas in 1971.
Haas tops a list of 30-somethings to reach the last eight after he joined a trio of 31-year-olds - Roger Federer, Tommy Robredo and David Ferrer - with his 84-minute demolition of Youzhny.
It is the first time in 44 years that four men on the wrong side of 30 have made it so far at Roland Garros but they have some way to go if they are to match Australia's Ken Rosewall, who won the last of his eight grand-slam singles title aged 37.
"We all just are smarter, about how you train, about how you eat, about how you do your recovery," said Haas, who was ranked second in the world in 2002 before injuries ravaged his body.
"The physical and fitness areas have improved a lot...I think that's why you see more people in the early, mid 30s doing well."
A year after scratching the word "SORRY!" into the Roland Garros clay during a third-round bashing by Ferrer, Youzhny suffered yet another meltdown and took out his frustrations by subjecting his racket to an almighty battering.
"Thwack" echoed around the arena nine times during a changeover as the Russian reduced his racket to a mangled mess by hammering it against his pristine white bench.
"It's bad luck for that racket," said an unrepentant Youzhny, who has form.
In 2008 he was left with a bloodied face after he repeatedly whacked his racket against his forehead and over the years has made his racket sponsors work overtime to keep up with his never-ending demands for replacements.
"He's done some crazy stuff in the past when he hit the racket on his head. I was not hoping for that again, but...that was fun to see. I love that stuff," said Haas, who set up a quarter-final against Djokovic. (Editing by Ed Osmond and Clare Fallon)