Olympic champion Zheng wins in Tokyo for third title of year
China's Zheng, the tournament's top seed, kept her cool to edge a tight first set despite two rain delays, before pressing home her advantage to take the second comfortably.
The win extended world number seven Zheng's impressive run of form this year after she became the first Chinese player to win an Olympic singles tennis gold when she triumphed in Paris.
She also won in Palermo and finished runner-up in Wuhan and at the Australian Open.
Kenin, the 2020 Australian Open champion, was in doubt after retiring from her doubles semi-final on Saturday with a leg injury.
The American played the final with heavy strapping on her upper leg but showed no outward signs of discomfort.
It was world number 155 Kenin's first final of 2024.
She was looking to end a title drought that stretches back more than four years.
Neither player was allowed to get into a rhythm early in the match as rain twice brought play to a halt in Tokyo.
The roof was eventually closed and Zheng edged ahead with the slimmest of margins to take the first set in a tiebreak.
Backed by a large contingent of Chinese fans, the 22-year-old took control of the second set early.
She hit a total of 16 aces in the match and closed it out when Kenin hit a return long.