USA passport, traveling from Vietnam to Guangzhou back to Vietnam on way back to USA.

I hold a USA passport.
I booked a RT ticket from SFO to SGN. In the middle of this trip I will be leaving SN and traveling to Guangzhou. I will be in Guangzhou less than 72 hours to visit a friend. I will then be flying back to SGN on my way back to USA. Do I need a Visa to enter Guangzhou? Mind you, the two RT flights were booked on different itineraries.