The American quartet of Tianna Bartoletta, Allyson Felix, English Gardner and Morolake Akinsoun clocked 41.77 to eliminate China from Friday's final.

It completed a surreal day for the reigning Olympic champions, who had earlier been left facing disqualifcation after a dropped baton exchange between Felix and Gardner in the morning heats.

The American women were thrown a lifeline however when replays showed that Felix had been knocked off-balance by Brazilian runner Kauiza Venancio as she prepared to hand off to Gardner.

The US team filed a protest and race officials duly disqualified Brazil -- but ruled that the Americans must run again, in a highly unusual solo flying lap -- to beat China's eighth fastest qualifying time of 42.70sec.

The decision angered Chinese officials but their protest was later dismissed by the International Association of Athletics Federations (IAAF).