Murray suffered only his fourth defeat of the year
World number four Andy Murray suffered a surprise 6-1 3-6 5-7 defeat to Argentine qualifier Juan Monaco in round two of the Rome Masters.
Murray, 21, who had a first-round bye, took the first set in 33 minutes but struggled on serve in the second.
The Scot came back from 4-2 down in the decider but clay-specialist Monaco broke for 6-5 and served out.
World number one Rafael Nadal beat Andreas Seppi 6-2 6-3, and Roger Federer overcame Ivo Karlovic 6-4 6-4.
"I'm coming here motivated, Rome is a very good tournament. I lost last year but it's impossible to win every year," said Nadal.
Monaco's victory over Murray secures him a meeting with Marin Cilic
in round three after the Croat overcame Belgium's Christophe Rochus 6-0 5-7 6-2.
I've had a great eight months and I was going to lose some time - I'm playing better this year than last year on clay and now I'll get ready for Madrid
However, Murray will still overtake defending champion Novak Djokovic as the world number three if the Serb fails to defend his title this week.
It is the first time that Murray has lost his opening match at a tournament since the 2008 Olympics and only his fourth defeat of 2009 - the other three coming against Nadal, in Indian Wells and Monte Carlo, and Fernando Verdasco at the Australian Open.
Murray's preparations for the French Open appeared in rude health when his clay-court season began with a run to the semi-finals in Monte Carlo, but he showed there is still much work to be done before Roland Garros gets under way on 24 May.
The British number one made just 57% of his first serves, compared to Monaco's 81%, and has now failed to advance beyond the second round on any of his four appearances in Rome.
"It wasn't my best match but I was still very close to winning," said Murray. "This year I've found ways of coming through when I haven't been playing well.
"On clay it's something I've got to work on more but I'm not going to be too disappointed.
Video - Murray exits Rome Masters
"I've had a great eight months and it (losing early) was going to happen some time. I've just got to realise that I'm playing better this year than last year on clay and now I'll get ready for Madrid."
Murray, who came out on top in his only previous meeting with Monaco in Miami, conceded just one game in the opening set.
Monaco could not find any rhythm and committed 16 unforced errors but in the second set it was Murray's turn to struggle and he was broken in game one to trail 2-0.
After seeing two break points go begging in game three, Murray earned a third chance and converted by firing a cross-court backhand deep into the far corner.
Murray held his serve to make it 2-2 and then had Monaco under pressure after winning the first two points of the next game, but he hit four shots into the net as Monaco reclaimed the lead in the set.
Breaks were exchanged again before Monaco opened up a 5-3 lead and he held serve to level the match.
"To be fair Monaco started playing much better," said Murray. "He made lots of mistakes in the first set.
"The court was very damp and heavy early in the match and in the end it was dry and quite slippery which made it a faster surface.
"He was able to dictate more of the points in the dried-out conditions."
Murray hurled his racket to the ground after netting to go 0-15 down in game three of the third set, and Monaco was able to break serve again.
At 4-2 down, Murray held serve and then broke back before holding for a 5-4 lead. Victory was a break away for Murray, but Monaco won eight straight points before comfortably serving out.
"Murray is still one of the best," said Monaco. "He has proved it throughout the year. He did very well in Monte Carlo. He is one of the most complete players on the circuit.
"He is adapting well to clay and he is a very tough opponent.
"At the beginning, I started very nervously. In the second set, I became calmer. I started playing slower. In the first set, I had hit the ball too hard and that wasn't the way to do it against Andy.
"I tried to tire him out in the second set. In the final set, I trusted my game. I had the chance and I played very well the last three or four games."
There was another surprise in the second round as world number five Nikolay Davydenko was beaten 7-5 7-6 (7-5) by Austria's Jurgen Melzer.
In an all-French battle, Richard Gasquet
beat ninth seed Jo-Wilfried Tsonga 7-6 (7-2) 6-4, while sixth seed Fernando Verdasco
defeated Mardy Fish 6-4 5-7 6-3 and eighth seed Gilles Simon
saw off Fabio Fognini 6-7 (6-8) 6-2 6-3.
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