Do I need a transit visa? US passport

I am currently working in Fukushima. I plan to visit the UK in December. My flight is with China southern and my departure flight will have a 14 hour layover in Guangzhou which I believe is covered by the 72 hour travel without visa. However, after the layover I leave Guangzhou for London and will have a 2 hour stop in Sanya(SYX). The aircraft and airline will remain the same. Will I need to get a transit visa for this stop or am I still eligible for the 24 hour visa-free transit?
Thank you.