For me, the most shocking upset was Leila Meskhi over Jennifer Capriati in the first round at the 1993 US Open. Although the upset is something we should have saw coming in hindsight (with the personal problems), at the time, Capriati was a 17-year-old star that was the Olympic Champion the previous year, and had made the quarterfinals of the first 3 slams in 1993, even making the finals at Toronto just a few weeks earlier.