Do I need a Visa If I Fly in to Hong Kong and want to go to Shenzhen or 72 hours?

I am a USA citizen will be flying into HK for one night then wanting to go to Shenzhen for 72 Hours then I fly out of HK to go to Japan. I all ready have my tickets showing my one way to Japan from Hong Kong and my leaving Japan 10 days later to come back to LAX. Do I need a visa for my time in Shenzhen? Thanks