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Olympic judo superstar Kayla Harrison says Ronda Rousey has given her "a lot of help" when it comes to making the transition into MMA -- but the real question ... would she ever take a fight against her old friend?

TMZ Sports spoke with 26-year-old Harrison -- a 2-time Olympic gold medalist in judo -- who says she trained with Rousey every day for 7 years as they fought their way to the top of the sport.

Now, with Harrison signing a contract with the World Series of Fighting -- she says she'll reflect on the advice she got from her old friend as she begins her training.