AWS savings plan and reserve instance

In short: The Reserved Instance (RI) takes priority over the Savings Plan. Since the RI would cover the whole of your 1 instance (I assume it meets the criteria of the instance you mentioned), the Savings Plan would not apply in this case.

In detail:

If you do happen to have both RIs and Savings Plans in your AWS Account, the application logic is as follows:

1. Zonal RI apply to the account that owns it; if there are RI hours left – it applies to the rest of Organization if you are part of one
2. Next, the non size-flexible Regional RIs (i.e. those for Windows) apply to matching usage on the account that owns it; if anything left is – rolls out to the rest of Organization
3. Then, size-flexible Regional RI applies to the account that owns it (smallest instance within the family first and going up to larger instances), then to the rest of Organization.
4. EC2 instance SP apply within the account that purchased it (starting from the usage that will result in highest discount and rolling down to usage with least discount) then to the Organization
5. Compute SP apply within the account that purchased it (starting from the usage that will result in highest discount and rolling down to usage with least discount) then to the Organization
6. Lastly, you would use On-Demand instances to cover the rest

Essentially, most restrictive takes priority and moves on to the more flexible options. You can read more examples here in our documentation: https://docs.aws.amazon.com/savingsplans/latest/userguide/sp-applying.html (Scenario 4 will be of interest to you).