Sanya Richards imposed her class on the field to head the qualifiers for Tuesday night's final and saw her path eased to a potential Olympic gold medal as two of her main rivals crashed out of the competition.
While the 23-year-old Richards dominated her semi-final, the 2007 World silver and bronze medallists, Nicola Sanders and Novelene Williams, surprisingly, failed to advance.
In the second of three semi-finals Richards looked a class apart, again demonstrating why she is a worthy favourite to become the first US athlete since Valerie Brisco-Hooks 24 years to take this title.
The world No.1 started aggressively, running down the stagger on Zambia's Rachael Nachula and Canada's Carline Muir in the first 100m.
Rounding the final bend she was briefly challenged by Russia's powerfully-built Anastasiya Kapachinskaya on her inside but she impressively maintained her form down the home straight to record 49.90.
Kapachinskaya, the 2003 World 200m champion, comfortably booked her spot in the final with 50.30 for second.
Novelene Williams failed to make an impact, however, and will not feature in the final.
The Jamaican who has struggled to reach top form this season finished third in a modest 51.06 and will now concentrate her efforts on the 4x400m relay. Christine Amertil of the Bahamas wound up fourth in 51.51.
In heat one World champion Christine Ohuruogu produced her best performance of the season to win the first semi-final in 50.14. The British athlete came with her familair late surge down the home straight to lift herself from fourth to first which suggests she will be the chief challenger to Richards.
She will be joined in the final by Shericka Williams. The 22-year-old Jamaican led coming off the final bend but did not panic when Ohuruogu swept past her and held on to second by 0.03 from the fast-finishing Tatyana Firova of Russia in 50.28. Firova also progressed as one of the two fastest losers.
Aliann Pompey, of Guyana, who led the field through 200m faded to fourth in 50.93.
Russia secured a full complement of three women in the final as Yuliya Guschina took victory in the third semi-final in 50.48. The bottle-blonde Guschina, who likes Kapachinskaya has achieved the bulk of her previous success in the 200m, was rewarded for her aggressive tactics and held off world No.1 Amantle Montsho of Botswana by 0.06.
Behind them Jamaican champion Rosemarie Whyte snatched third in 50.63 to claim the second fastest loser spot for the final. But Sanders missed out after finishing fourth in 50.71. The British athlete rounded into the homestretch even with Guschina, but ran out of gas in the latter stages and faded to fourth in 50.71.
Richards, however, will be the only US athlete in the final. Mary Wineberg finished fifth in 51.13 in heat one and DeeDee Trotter seventh in 51.87 in heat three.
Steve Landells for the IAAF