The Spaniard, struggling with an injured left wrist since November, appeared to meekly concede the latter stages of the match in the 6-4, 6-1 defeat, losing the last game to love.
In contrast Venus Williams, the No 3 seed, needed less than an hour to dispatch the ninth-seeded Anna Kournikova 6-2, 6-2. Williams, who has already won three tour events in 1999, mixed deft drop-shots and the occasional net rush with her customary power game.
Her sister, the No 6 seed, Serena Williams, overpowered the 13th-seed, Irina Spirlea, 6-2, 6-3. Serena has now reached the final of her last three events, winning two of them.
Plischke spoke modestly of her victory over Sanchez-Vicario. "Arantxa obviously was not in her greatest form today," said Plischke, who is ranked 38th in the world, "but I knew I had to stay aggressive." Plischke recently parted with her coach, Erik Van Harpen, and came to the Italian Open alone - which she said has helped her game. "It gives me more time to think," she said. Her quarter-final opponent will be France's Amelie Mauresmo, who came back from a set down to oust Patty Schnyder 5-7, 6-2, 6-4.
The defeat casts further doubt over Sanchez-Vicario's chances of defending her singles title at the French Open, which begins on 24 May. Most of the Spaniard's career highlights, including three of her four career Grand Slam titles, have come on clay, a slow surface conducive to her baseline game.
However, her form on clay has been uneven this season. She lost her first match on the surface at Hilton Head, South Carolina, in March, beat a weak field for the tournament title in Cairo and lost last week in the semi-finals at Hamburg.
Mary Pierce, the fourth seed, also advanced in Rome, battling back to eliminate Conchita Martinez, 4-6, 6-0, 7-6, in a match that lasted nearly three hours. Pierce beat Martinez in the 1997 Italian Open final, ending the Spaniard's run of four consecutive titles.
In a minor upset the 11th seed, Dominique Van Roost of Belgium got past the seventh-seeded Nathalie Tauziat, a Wimbledon finalist last year, 6-4, 7-6.
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