1998 was Kournikova's best year on tour. Her biggest win was an upset of Steffi Graf on the grass courts of the Eastbourne tournament that year. Her best tournament was the '98 Miami tournament where she upset 4 top 10 players in succession (Seles, Martinez, Davenport, and Sanchez-Vicario) before falling to #11 Venus Williams in 3 sets in the final.
After having a great year in '98, the media became fixated with her sex symbol status and her results nose-dived thereafter.
Her best slam result was 1997 Wimbledon where she recahed her only slam semi (falling to Hingis).
Kournikova's had some nice wins over quality opponents throughout her career: Davenport at her "home tournament" in San Diego, Seles, Hingis, Majoli on clay (while she was defending French Open champ), Sanchez Vicario on clay, Graf on grass, Martinez, Clijsters, Capriati, Mauresmo, and Dementieva. She's never beaten either Williams sister, but she did take them to 3 sets on occasion.
Kournikova wasn't a bad player, she just couldn't control her nerves when it counts. Mostly, it was her serve that deserted her in big moments. She had a lot of pressure on her to win a title and justify the hype