(adds quotes, Kuznetsova win)
* Nadal toys with Lajovic to march on
* Ferrer next for eight-times champion
* Halep, Petkovic and Errani advance in women's draw
PARIS, June 2 (Reuters) - Concerns that Rafa Nadal's reign as king of Roland Garros might be in jeopardy due to an aching back proved wide of the mark on Monday as the world number one led a mini Spanish charge into the French Open quarter-finals.
On the day that Spanish King Juan Carlos abdicated his throne, there was no danger of his compatriot doing the same in Paris as the eight-times champion produced a 6-1 6-2 6-1 demolition job on Serbian Dusan Lajovic.
Next up will be a man who beat Nadal the last time they faced each other across a net - fellow Spaniard David Ferrer.
Ferrer stalled Kevin Anderson's bid to become the first South African man in 47 years to reach the last eight of the claycourt major with a 6-3 6-3 6-7(5) 6-1 win.
The top half of the men's draw could have become an all-Spanish affair if Wimbledon champion Andy Murray and elastic-limbed Gael Monfils had not played spoilsport.
Seventh seed Murray and Fernando Verdasco walked out under sunny skies looking like clones - both kitted out in near identical canary yellow shirts and black shorts.
Both players turned the air blue during a heated third set but it was Murray who buzzed around Court Suzanne Lenglen, stinging Verdasco with vicious winners for a 6-4 7-5 7-6(3) triumph.
While the linecall dispute was over in a flash after Murray gamely conceded the point, Verdasco blamed umpire Pascal Maria for fanning the flames.
"Pascal is very peculiar. Several times I had a bad experience with Pascal Maria. He's not the kind of umpire I get along with. I can tell you that," Verdasco said.
"He is an umpire that many players remember vividly and not because of his qualities."
Frenchman Monfils kept the home fires burning with a 6-0 6-2 7-5 win over yet another Spaniard, Guillermo Garcia-Lopez.
But no one feels more at home at Roland Garros than Nadal.
The top seed, who said he had to slow down his serve in his previous match after being troubled by back pain, left Lajovic with a sore head and aching joints as he went on a rampage to go 5-0 up in the first set before rattling off 17 straight points at the start of the second.
The winners flying off Nadal's racket appeared to leave everyone in such a trance that the umpire even fluffed his lines at one stage - telling the players 'to replay the point' in English before sheepishly repeating the instruction in French - drawing a rare smile from Lajovic.
With enigmatic American pop singer Prince watching from the stands, it did not take 83rd-ranked Lajovic too long to discover why beating Nadal at Roland Garros is one of the hardest riddles to crack.
On the eve of his 28th birthday, a screaming forehand winner allowed Nadal to take his formidable French Open win-loss record to 63-1 and just three wins away from again sinking his teeth into the Musketeers' Cup.
The result enabled Nadal to cross a major psychological hurdle as the last time he was going for five in a row in the French capital, he was beaten in the fourth round by Robin Soderling.
Another encouraging sign for Nadal was that his serve had once again picked up speed and there was no evidence of any discomfort.
"My back can be pretty unpredictable," was all Nadal was willing to say when he was quizzed about the subject.
While Nadal being in the final of the French Open is one of the most predictable sights in tennis - his only blip was in 2009 - Ferrer made it that far for the first time only 12 months ago.
The odds of him repeating that run were boosted by his defeat of Nadal in the quarter-final of the Monte Carlo Masters weeks ago but as Ferrer knows only too well, outclassing his celebrated rival on the regular tour is completely different to toppling him in a best-of-five set match on red dirt.
"Tactically, I will have to be perfect. I hope that I will instil some doubts in Rafa's mind, but if we both play at our best level, he will be the better player," summed up Ferrer, who has beaten Nadal only six times in 27 meetings.
While three of the world's top five men are still alive, Romanian Simona Halep was the only seed among the top six women to reach the last eight with a 6-4 6-3 win over Sloane Stephens, whose exit ended American interest in the singles.
She will next face 2009 champion Svetlana Kuznetsova.
Andrea Petkovic's decision to hold off trading in her tennis racket for a career in journalism paid off as she reached a grand slam quarter-final for the first time in three years with a 1-6 6-2 7-5 win over Dutch qualifier Kiki Bertens.
Hobbled by back, ankle and knee injuries that forced her out of the tour for several months over the past two years, the German considered quitting tennis in 2013 to try her luck as a magazine journalist.
But having put that plan on the backburner for now, she will be eager to pen a happier tale this week by reaching her first grand slam semi-final.
She will next face 2012 runner-up Sara Errani, who continued the cull of top 10 players with a 7-6(5) 6-2 win over sixth seed Jelena Jankovic.
"It's a big disappointment, especially when you see who is left in the draw. With all the seeds who left the draw early, it opened up... (but) I did not do so well," shrugged Serbian Jankovic. (Editing by Justin Palmer/Mark Meadows)