"It is with deep regret that I have to withdraw from tomorrow's Boston Marathon", the 26-year-old said in a statement. After winning the New York Marathon in November - becoming the first American woman to take the title since 1977 - Marblehead, Mass., native Flanagan is a favorite in Boston.
On Sunday night, just hours before the 2018 Boston Marathon, elite race sponsor John Hancock announced that Jordan Hasay, the top returning American woman from 2017, has withdrawn from this year's race.
Later that year, the former University of OR star followed with a third-place finish at the Chicago Marathon, with an improved time of 2:20:57.
The cloud on Hasey's horizon, however, is that she has reportedly been suffering from a foot injury.
The latter runner, 33, finished third in the 2016 New York City Marathon a year after missing a World Championship orld bronze medal in the 10,000m after her premature celebration allowed compatriot Emily Infeld to pass her on the line.
Hasay, New York City Marathon victor Shalane Flanagan and fellow Americans Desiree Linden and Molly Huddle had been expected to challenge Kenyan defending champion Edna Kiplagat to end the nation's 33-year wait for a home victor in the women's race.
Although Huddle's time in her marathon debut was on 2:28:13 she has the competitive temperament to make her second attempt at the distance even more memorable.
It's been five years since the Boston Marathon bombings that killed three people and injured several hundred others.
Galen Rupp wins the Chicago Marathon in October 2017.
Shalane Flanagan becomes the first US woman since 1977 to win NYC Marathon.
Meanwhile, Kirui aims to repeat last year's success.