She has represented Bulgaria in twelve Chess Olympiads from 1992 to 2014.
In 1989, Stefanova won the under-10 girls’ section at the World Youth Chess Championships in Aguadilla, Puerto Rico. In 1992, she played, at the age of 13, in her first Chess Olympiad in Manila, Philippines. In the same year she became European under-14 girls’ champion at the European Youth Chess Championship in Rimavská Sobota. Stefanova won the Bulgarian women’s championship in 1995.
She tied for fourth place in the 4th Hawaii International Chess Tournament in 1997 scoring 7 points out of 10 games. Thanks to this result Stefanova achieved her first Grandmaster norm. In January 1998, her FIDE rating broke into the top ten of women worldwide. She played in the open section at the 2000 Chess Olympiad. In 2001, Stefanova finished equal first (second on countback) in the 19th Andorra Open.
In June 2002, she won the 3rd European Individual Women’s Championship in Varna. Stefanova was awarded the title of Grandmaster at the FIDE Presidential Board meeting in Doha in July 2002. At the end of July 2002, she won the Wismilak International Chess Tournament, a category 8 (average 2446) round-robin tournament in Surabaya, Indonesia, scoring 9.5/11 with aperformance rating of 2750.
In 2008 she won the North Urals Cup in Krasnoturinsk, Russia, and the women’s individual rapid tournament of the 2008 World Mind Sports Games in Beijing. In 2012, Stefanova won the Women’s World Rapid Chess Championship. She was the runner-up in the Women’s World Chess Championship 2012, losing to Anna Ushenina in the final on the tie-break.