Korda did beat Rios at the '98 AO. Korda tested positive for nandrolone, I believe, at Wimbledon later that year. His test was clean at the AO, so losing the title was never under consideration. Rios never won a major.
What gm3106 posted is very true. However, my understanding is this was ephedrine that showed up. It is in cold medicines. I believe it is banned because it can give an immediate and temporary jolt in performance. I don't think it can benefit a player longterm. I remember when that gymnast, Raducan (?), was stripped of her gold medal because of ephedrine. She had a cold and took cold medicine during the all-around competition, but they let her keep her other gold medal for the team competition, because that test was clean. Therefore, the benefit must be very short-lived, unlike anabolic steroids.
What would be the purpose of Kuznetsova needing an extra jolt at an exhibition? I bet she had a cold. The stuff would be out of her system quickly, sort of like caffeine, which I believe high doses of that are also banned.