Nimblewill is a challenging weed as it is a grassy perennial.
Take a look at our page on it: http://extension.umd.edu/hgic/nimblewill
How much of it do you have?
There are a couple of options. The chemical listed in the page can be put over tall fescue. The other option is to spot treat the sections you have with a non-selective herbicide containing the active ingredient glyphosate (which kills anything green you spray it on), wait for dieback, and then re-seed those areas.
Now is the perfect time to get started doing this.
Here is our page which helps you with the steps:http://extension.umd.edu/hgic/lawns/lawn-renovation
Be sure to look at the link to the HG102 publication at the bottom of the page, which is extremely helpful and can answer all questions on how to do this for best success.