Tarmoh was originally named as the winner of the final spot on the US team for London but officials declared a dead heat after reviewing the photo finish.
"I was robbed, I went to bed so happy then I woke up to do something that I did not want to do at all," Tarmoh said after USA Track & Field (USATF) announced eight days after the race that there would be a runoff.
"I was pushed into a corner. They said if you don't make a decision you give your spot up."
Since only three athletes can be named in each event to an Olympics team, the tie needed to be broken but U.S. officials did not have a procedure in place.
They developed one a week ago in consultation with the US Olympic Committee but allowed the athletes to wait until after Saturday's 200 to decide whether there would be a runoff or a coin flip for the final spot.
"That is what it is, I think we are both not feeling our greatest. That is my main concern," Felix said less than 24 hours after she ran the sixth fastest 200 metres ever in a US final in which Tarmoh finished fifth.
"If anything feels off whatsoever I am just going to speak up and have to pull out of it."
World champion Carmelita Jeter won the 100 with Tianna Madison second.
Felix could wind up running four events in London if she wins the 100 runoff. She also hopes to compete in the 4x100 and 4x400 metres relay in addition to the 200.
Tarmoh also is on the US team regardless of Monday"s race as a member of the 4x100 relay pool.
USATF president Stephanie Hightower said she was pleased with the athletes' decision to run.
"They both feel they deserve a spot on the team and they're willing to do it in a competitive manner and a competitive fashion," she said. "I think it's good for the sport and I think it's good for them to be able to show their level of competitiveness and passion and drive for what they've been working so hard to accomplish."