Where are you placing your foot on the pedal?

Whe I rode my hybrid last year with tennis shoes and platforms, i used to place my toes towards the back of the pedal. I was mainly using my toes to do all of the work. THis caused horrible pain and numbness, until someone told me to move the pedal more under the ball of my foot. Ohhh! What a difference.

I am not sure of the exact position you should use, but maybe try a few different spots.

Even after this mostly corrected the problem, I would still get slightly numb feet on longer rides. I would have to take my feet off the pedals every couple of miles and move them around.

Also, are your tennis shoes wide enough in the toe box? Your feet might be swelling while you ride.