ATHENS - Tonique Williams-Darling has claimed the Bahamas' first individual athletics gold by blasting to the women's 400m title.
Russian Natalya Antyukh took the bronze in 49.89s.
The Bahamian's triumph confirmed her dramatic rise to prominence this year, during which she has repeatedly shattered her personal best.
Guevara, unbeatable in 2002 and 2003 but who was hampered by an Achilles tendon injury early this season, seemed to have the advantage in lane three as she drew level with her rival with 100m remaining.
But Williams-Darling, who specialises in lightning starts and a strong first 200, found new strength after the Mexican edged a metre ahead, and pulled back in front in the final 50.
In the end, the race went to form. Williams-Darling had beaten Guevara twice in the run-up to the Games, also ending her 28-race winning streak stretching back to the 2001 world championships.
But Guevara's performance was a brave one after her troubled season, which cost her more than two months of training.
The 27-year-old former basketball player had hoped to become Mexico's first female athletics Olympic gold medallist after winning at the Paris world championships last year.
The Bahamas' only previous gold came in the women's 4x100m relay in Sydney.
