Pierre-Jean Vazel opened this up on twitter and it's a good question.
The question almost seems preposterous at first but Caster Semenya closed in 13.6.
Is that the fastest ever? We have an article here with video of the finish:
http://www.letsrun.com/news/2016/05/final-100m-caster-semenyas-158-26-800m-fastest-ever/
Here's our commentary on some otger final 100m:
We went and watched Semenya’s 1:55.45 800m PR from the 2009 Worlds and her final 100 couldn’t have been faster than 14.5. We watched world record holder Jarmila Kratochvílová’s 1:54.68 World Championship win in 1983 and can only guess where the 100m to go mark is from watching the 100m, but guess she closed in no faster than 13.9. The fastest 800m in the last 30 years was Pamela Jelimo’s 1:54.01 in Zurich in 2008. We can’t see the 100m mark in that race either but don’t think she closed faster than 14.4.