In snapping Swiatek's 25-match winning streak at Roland Garros, Australian Open runner-up Zheng became the first Chinese player to reach the singles final at an Olympic tournament.
The country's previous best singles result at the Olympics was Li Na finishing fourth in the 2008 Beijing Games.
Zheng, who did not even have to bring her A-game against a badly off-key Swiatek, will face either Slovakia's Anna Karolina Schmiedlova or Croatia's Donna Vekic for the gold medal. Swiatek will have to console herself with a match for a bronze medal.
With so many big names missing from the Olympic singles and with several seeds having already fallen, Swiatek had looked odds on to add the Olympic gold to her five Grand Slam titles.
She held a 6-0 career record against Zheng, but the 23-year-old Swiatek picked the worst possible time to produce one of her worst displays of the season, spraying 36 unforced errors.
Swiatek was never in the opening set as she looked uncomfortable in the hot and humid conditions, struggling to find her range with shots often missing the lines by metres.
After a bathroom break to re-set she seemed to have regained control in the second set as she went 4-0 ahead.
But it proved a false dawn as the errors returned and Zheng took full advantage to claw her way back.
Swiatek dropped serve at 5-5 and Zheng then kept cool as she served for victory -- banging a first serve on to the line on match point that her opponent thought had landed out.
A quick check from the umpire confirmed Swiatek's fate.
The men's quarter-finals are also on the schedule on Thursday with second seeded Spaniard Carlos Alcaraz first up against American Tommy Paul.
Serbian top seed Novak Djokovic continues his bid for long-awaited Olympic gold against Stefanos Tsitsipas while defending champion Alexander Zverev faces Italy's Lorenzo Musetti.
The other men's last-eight clash is between Canada's Felix Auger-Aliassime and Norway's Casper Ruud.