The Scottish middle distance runner finished second in the first semi-final, a stride behind Jamaica’s Natoya Goule.
The top two from each of the three semi-finals qualify for Tuesday’s final along with the two fastest losers.
Goule’s winning time was 1min 59.57sec while Reekie clocked 1:59.77.
It was an impressive performance from Reekie who tucked in behind Goule early on. The pair pulled clear of the rest of the field on the final bend.
In the build-up to the Games, Reekie, 23, ran a new personal best of in 1:56.96 at the Diamond League meeting in Monaco.
Keely Hodgkinson also advanced to the final of the 800m as the winner of her semi-final, beating Chunyu Wang of China in a photo finish, meaning three Brits still contest the final as Alex Bell made it in as one of the fastest third-place finishers.