China’s Guan Chenchen, a star on beam, performed an excellent routine on that apparatus. She scored a 14.933, the highest mark after two of five subdivisions. Chenchen will be a medal favorite in the final, and she might have the best chance of disrupting Simone Biles’s ability to win five gold medals. Biles is expected to win in the team and all-around competitions, as well as in the vault and floor finals. Biles has a difficult beam routine, but that’s where a competitor such as Chenchen could threaten. At the 2016 Games, Biles won four golds and a bronze on beam.