Argentina's Paola Suarez beat Russia's Anna Kournikova 2-6, 6-2, 6-4 in the quarter-finals of the rain-plagued Bausch and Lomb Championships in Amelia Island, Florida.
Suarez, who upset second-seeded Serena Williams earlier in the week, struggled in the first set on Saturday, failing to hold serve on all four attempts and committing seven double faults and 23 unforced errors.
But she took control in the second set, using deep, accurate ground strokes to keep Kournikova off balance.
"I think the key was that I continued to get better during the match," Suarez said. "I did not play well in the first set, but even then I thought I was getting better all the time.
"I started to think more about each shot, exactly where I wanted to place it. I tried not to think about winning the match, I was just concentrating on winning one point at a time."
Kournikova, frustrated by her play and several calls, slammed her racquet into the clay surface late in the match, drawing a code violation from the chair umpire.
"I was probably playing too defensive in that second set," Kournikova said. "She (Paola) kept a lot of her shots low, so it was tough to come to the net on her."
Suarez will play defending champion Monica Seles on Sunday in the semi-finals. Seles continued her mastery over Spain's Arantxa Sanchez-Vicario, winning 6-1, 6-3.
"I just played extremely well today, and that really felt good," said Seles, 18-3 against Sanchez-Vicario. "I was trying to put a lot of pressure on her on her returns. It seemed that everything was going my way today."
Russia's Elena Likhovtseva will play Spain's Conchita Martinez in the other semi-final. Likhovtseva beat top-seeded Mary Pierce of France 2-6, 6-4, 6-4, and Martinez beat Austria's Barbara Schett 6-4, 6-3.
Results from the $535,000 Bausch and Lomb Championships with seeds in parentheses:
Paola Suarez (Arg) def. (7) Anna Kournikova (Rus) 2-6, 6-2, 6-4. (3) Monica Seles (US) def. (6) Arantxa Sanchez-Vicario (Sp) 6-1, 6-3. (4) Conchita Martinez (Sp) def. (8) Barrbara Schett (Aut) 6-4, 6-3. (12) Elena Likhovtseva (Rus) def. (4) Mary Pierce (Fr) 2-6, 6-4, 6-4.
Doubles Quarter finals
Barbara Schett (Aut) and Patty Schnyder (Swit) def. (7) Nicole Arendt (US) and Patricia Tarabini (Arg) 6-2, 7-5. Lisa Raymond (US) and Rennae Stubbs (Aus) def. Virginia Ruano Pascual (Sp) and Paola Suarez (Arg) 2-6, 6-4, 6-4 (6) Amanda Coetzer (SA) and Ai Sugiyama (Japan) def. Surina de Beer (SA) and Liezel Horn (SA) 7-6 (1), 6-3