* Berdych, Del Potro ousted
* Defending champion Nadal in solid form (Add quotes, late result)
By Gregory Blachier
MONTE CARLO, April 18 (Reuters) - A string of errors put world number two Andy Murray out of the Monte Carlo Masters on Thursday when he lost 6-1 6-2 to Swiss Stanislas Wawrinka in the third round.
Fourth seed Tomas Berdych and Argentine Juan Martin Del Potro, seeded five, were also beaten at the first big claycourt event of the season, while Rafael Nadal calmly continued his bid for a ninth consecutive crown with a 6-2 6-4 victory over Philipp Kohlschreiber of Germany.
Top seed Novak Djokovic, who made a late call to participate after recovering from a sprained ankle, overcame a second opening set scare of the week to move past Juan Monaco of Argentina 4-6 6-2 6-2.
Briton Murray, who has repeatedly said he needs time to find his feet on the red clay, looked frustrated on court and picked up only eight points on serve in the first set.
"I made a lot of mistakes - 24 unforced errors is far too many ... that's where half the points went," Murray told a news conference.
"I started hitting the ball shorter, then he was able to dictate the play," added Murray, who made the last eight last year and is now set to slip to third behind Roger Federer in the ATP rankings.
The 17th-ranked Wawrinka, who had won their two previous encounters on the surface, reached the quarter-finals for the second year in a row and will now face Frenchman Jo-Wilfried Tsonga, who overwhelmed Austrian Juergen Melzer 6-3 6-0.
Czech Berdych, a semi-finalist last year, was humbled 6-4 6-2 by Italy's Fabio Fognini who also beat him in the first round here in 2009.
Berdych was broken four times overall and never got into a rhythm.
Fognini will now face seventh seed Richard Gasquet of France who beat Croatian Marin Cilic, seeded nine, 7-5 6-4.
Defending champion Nadal notched up his 44th win in a row in the principality and set up a quarter-final encounter with Grigor Dimitrov of Bulgaria.
German Kolschreiber did not pose a big threat to Nadal, who has already won three titles this year following his return from a seven-month knee injury layoff.
The Spaniard pleased the cheering crowd with a solid display on the sun-baked Centre Court that has been his stage since 2005.
"The match started with a lot of intensity for both of us and I was able to have the break. After that, I was comfortable with my serve," Nadal said.
"The second set, I think the intensity went down a little but I was in control."
Dimitrov, who at 21 is making his debut in the Monte Carlo main draw, beat German Florian Mayer 6-2 6-4.
World number one Djokovic seemed uncomfortable when he checked his right foot after losing the first set to 14th-seed Monaco, only to fight back with heart and go through.
The Monaco-based Serbian, who injured his ankle while on Davis Cup duty earlier this month, withstood the long rallies and rediscovered his sharpness to eventually prevail.
"The feeling was quite similar as yesterday," Djokovic said.
"I'm experiencing something new this week. I have to triple the effort I put on court because there are so many things I'm thinking about. So very proud that I managed to stay mentally tough in the first two matches."
Last year's finalist Djokovic will take on Finland's Jarkko Nieminen, who eclipsed Del Potro 6-4 4-6 7-6 as the sun set over the secondary Court of Princes. (Editing by Clare Fallon and Pritha Sarkar)