72 hr visa free stay

I am an American passport holder. I have a flight from Manila, Philippines to San Francisco, CA on China Southern Airlines, with a 2 day visit to Xi'an. Question is, the airline transits through Guangzhou for a switch in aircraft. Both Guangzhou and Xi'an have 72 hr free stays. I plan to stay the 2 days in Xi'an. The flight onwards to San Francisco takes me back to Guangzhou fr Xi'an and onwards to the US. Will the 72 hr free stay apply to my port of entry (Guangzhou) only or will it apply to my stay in Xi'an also? Or do I have to get a eg. tourist L type entry permit?.....Confused