With world numbers one and two Roger Federer and Andy Roddick already ousted from the Athens Games, third seed Moya was tipped for the title but the Spaniard was out-gunned 6-2, 7-5 by an effervescent Massu.
"I made many errors," Moya said. "It was one of my last chances to win an Olympic medal."
In contrast, Massu was euphoric.
"That was one of the best matches I have ever played," he said. "Now I want to go on and win the tournament."
Were he to do so, he would become the first Chilean athlete to win Olympic gold.
In any case, Chile are guaranteed a medal at the Athens Games. At worst, both Massu and compatriot Fernando Gonzalez could both lose their semi-finals and would then play off for the bronze.
Massu plays American Taylor Dent in the semis and Gonzalez, who ousted eighth seed Sebastien Grosjean 6-2, 2-6, 6-4 meets Mardy Fish, the American who beat Russia's Mikhail Youzhny 6-3, 6-4.
WOMEN PROSPER
While the top men have faltered at the Games, the leading women continue to prosper.
Favourite Justine Henin-Hardenne led from the front.
The Belgian beat Mary Pierce -- conqueror of Venus Williams in the last round -- 6-4, 6-4 to set up a semi-final against third seed Anastasia Myskina.
"It's going to be a tough mission for me," Henin smiled.
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French Open champion Myskina thrashed Italy's Francesca Schiavone 6-1, 6-2 in less than an hour.
"I thought it was going to be tougher," the Russian said. "I didn't expect it to be so easy."
In the bottom half of the draw, second seed Amelie Mauresmo defeated Russia's Svetlana Kuznetsova 7-6, 4-6, 6-2 and will play Alicia Molik for a place in the final. Molik defeated Japan's Ai Sugiyama 6-3, 6-4.
Mauresmo is looking forward to her clash with the Australian.
"She beat me like two or three weeks ago," the Frenchwoman said. "So I have a good revenge to take ... It's going to be a good one. I am playing well ... You know, physically and mentally it was a tough match for me today and I got over it."
Molik will not be overawed, however.
"I've got as good a shot as anyone. I've just got to keep going," she said.
Martina Navratilova could not manage to keep going, though, and at the age of 47 her Olympic odyssey came to a shuddering halt.
Partnering Lisa Raymond, she lost in the quarter-finals of the women's doubles to Japanese pair Ai Sugiyama and Shinobu Asagoe 6-4, 4-6, 6-4.
