In a twist to a marquee event, Allyson Felix and her teammates on the U.S. women's 400-meter relay team get a second shot to make the final, after successfully arguing that other runners made them drop their baton at the Summer Olympics Thursday.

Track and field's international governing body, the IAAF, agreed with the U.S., setting up an unusual event tonight — 7 p.m. in Rio and 6 p.m. ET – when the Americans will run a race alone on the track at Olympic Stadium.

To reach the final, the U.S. relay team will need to beat the time of 42.70 seconds that was run by China's team. So far this season, the best recorded time for the Americans is 42.61.

Here's how NPR's Tom Goldman describes the problematic part of this morning's preliminary heat, in which the U.S. team ran in the second lane, next to Brazil's team in lane 3:

"Felix was about to hand off to English Gardner for the third leg when, Felix says, she was bumped by a competitor. Felix stumbled and tried to throw the baton to Gardner and it fell to the track."Brazil and Kazakhstan were disqualified — the first for obstruction, and the other for lane infringement."

In reviewing the U.S. appeal, officials reviewed video of the race that showed a Brazilian runner who was poised to receive her team's baton was on the edge of her lane — and that she swung an elbow out that hit Felix in the same arm in which she carried the U.S. baton. Tonight, the American women will again run in lane 2.

Noting that China, the eighth-place qualifier, now sits on the bubble and will wait for hours to learn whether its spot in the final is safe, Tom adds, "Not surprisingly, China has filed a protest."

China's runners didn't compete directly with the Americans this morning, as they were in the other qualifying heat.

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