Britain's Andy Murray suffered a shock 7-5 6-3 defeat by Robin Haase in the opening round of the World Tennis Tournament in Rotterdam.
Murray is 84 places above Haase in the world rankings
The world number 10 conceded the first set when Haase broke his serve in the 12th game to take the advantage.
And the Dutchman held on for the win after breaking Murray's serve in the eighth game of the second set.
Meanwhile, top seed Rafael Nadal reached the second round by beating Russia's Dmitry Tursunov 6-4 6-4.
Murray refused to blame his defeat on fatigue, saying: "I wasn't tired physically or mentally, I felt decent."
The loss came just three days after the Scot's second title of the year at the Open 13 tournament in Marseille, a victory which propelled him back into the world's top 10.
"I had trouble with my return game," said Murray. "The court here is much slower than in Marseille."
Murray had made a positive start at the Ahoy Indoor Arena, but failed to make the most of his two early break points as his 20-year-old opponent fought back to take the first game of the match.
The turning point came in the 12th game as Haase - ranked 84 places below Murray in the world - upped his return game to earn three set points against serve.
Although Murray managed to hold on, he could do nothing about the fourth as Haase smashed a winning forehand to take the first set.
The second set went with serve until the eighth game when Murray's inconsistent second serve was punished by Haase, who took a 5-3 lead and served out to set up a clash with Germany's Michael Berrer.
"This is just one of those matches you can afford in tournaments like this," Murray added. "It will not affect your ranking like Master Series or Grand Slams.
"Now I will have some days off before I start preparing for Dubai and the Indian Wells and Miami stretch. It still has been a good start of the year."
Also out is world number 19 Lleyton Hewitt, who was beaten 3-6 7-6 7-6 by Italian Andreas Seppi.
The Australian, who won the tournament in 2004, dominated the first set from the baseline but struggled in both tie-breaks, which he lost 7-4.
"It is frustrating if you have chances like this to proceed in the tournament," he said.
"I just need a bit of luck and no injuries and I am sure I can work back towards the top five."
Seppi will next play Nadal after the Spaniard took just 90 minutes to secure victory over Tursunov before an 8,000 sell-out crowd.
The hard-hitting Tursunov did not earn a single break point against Nadal, who always looked comfortable but played down the ease of his progress.
"It was a tough first round," said Nadal, who was making his debut in Rotterdam.
"Tursunov plays very aggressive and has a lot of power. I knew I had to serve well to keep on top of him.
"Seppi beat Hewitt so he will come into the match with big confidence, I have to be ready."