Alexandrova has been called up to replace Keys after Andreeva, currently ranked as world No 9, decided against participating because of lacking fitness to play in today's matches, according to the Women's Tennis Association.
Current coverage have highlighted concerns about how players are facing challenges by the WTA.
Welcome to another day at the WTA Finals. Although both matches offer thrilling action, the primary attraction showcases Iga Swiatek facing off with Amanda Anisimova.
Their last meeting at the Wimbledon Championships earlier this season was remarkably one-sided, with Swiatek achieving a commanding double bagel win. However, the American player made an impressive recovery just weeks later, defeating Swiatek in straight sets during the US Open quarter-finals.
Today's match represents the final installment of their competitive rivalry, with the winner advancing to the semi-finals in the Saudi Arabian capital.
Earlier in the day, Rybakina will face Ekaterina Alexandrova instead of Madison Keys. Keys pulled out due to health issues in what constitutes a meaningless match, paving the way for the world No 10 to participate. Rybakina aims to maintain her unbeaten streak, having previously defeated both Swiatek and Anisimova to secure her last-four position.