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Don't slam the door on a possible Cormier vs. Miocic rematch -- with D.C. telling TMZ Sports he's open to the idea ... AFTER he fights Brock Lesnar.

Remember, after Daniel Cormier defeated Stipe Miocic at UFC 226 earlier this month ... Lesnar stormed the cage and hyped a Cormier vs. Lesnar superfight.

Miocic -- the former heavyweight champion of the world -- is furious claiming he's EARNED a rematch with Cormier before Lesnar gets a shot ... considering Brock hasn't even fought since 2016.

So, when we saw Cormier at the ESPYs on Wednesday ... we asked how he felt about Stipe's gripe.

Of course, the clock is ticking considering Cormier is 39 years old and wants to retire at age 40. His birthday is in March.

Tick tock ...