Earlier on Friday, Gemili's time of 20.17s in the heats was the fastest by a Briton since 2005. His previous PB had been 20.30, meaning he has lowered it by almost a third of a second in one afternoon.
The 19-year-old feared he had showed his hand to early, but in the afternoon semi an astonishing finish saw the Londoner win through in the fastest time ever by a European U19 athlete, and the second fastest British time behind John Regis' 19.94.
Defending champion Usain Bolt comfortably won his semi-final in 20.17s.
The final is at 5.10pm UK time on Saturday.