It is another step in the Olympic gold medallist's injury comeback, following a gritty showing at last month's US Open where she reached the quarter-finals.
Zheng, a former world number four, had struggled with her form since needing elbow surgery in 2025 and is a wildcard entry in Beijing.
"I had a great performance tonight," Zheng told the crowd afterwards.
But she added: "I would not say I'm back. I would say the best version of me is still coming in the future."
Zheng dropped just three points as she eased into a 3-0 lead, punching winners into the corner with searing accuracy.
Her game faltered afterwards as she needed to warn the crowd to "stop speaking" before Mertens levelled to 4-4, but held her nerve to win the first set on the first time of asking.
She carried that momentum into a dominant second set, closing out the match with 43 winners across the one-hour-and-a-half affair.
Earlier, Russian teenager Andreeva snagged the first spot in the final, brushing aside Nikola Bartunkova 6-2, 6-2.