The Frenchman lost his serve on a double fault in the third game of the second set.
French Open champion Gaudio, seeded second, broke his opponent's serve in the opening game of the first set before taking a 5-1 lead. Lopez made 16 unforced errors in the 25-minute set.
In other quarterfinals, fourth-seeded Tommy Robredo had little trouble against Italy's Davide Sanguinetti who, at 32, is the tournament's oldest player.
In the women's tournament, second-seeded Dinara Safina reached the semifinals by beating American Jill Craybas 6-4, 6-2.
Argentina's Gisela Dulko, seeded third, beat ninth-seeded Jie Zheng of China 6-2, 6-1, while fourth-seeded Na Li took less than an hour to overcome Dally Randriantefy of Madagascar, 6-3, 6-1.
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