Marozsan stuns Ruud to reach Shanghai Masters last eight
The unseeded Marozsan won 7-6 (7/3), 3-6, 6-4 and will face Poland's 16th-seeded Hubert Hurkacz or home player Zhang Zhizhen in the last eight.
The 24-year-old sealed the deal in two hours and 38 minutes with a stinging forehand down the line on his second match point.
The Shanghai event, which is back on the ATP calendar for the first time since the Covid pandemic, has witnessed several shocks.
Second-seeded Daniil Medvedev was dumped out on Sunday by the 26th seed Sebastian Korda and Stefanos Tsitsipas, the fourth seed from Greece, lost on Monday.
It paves the way for top seed Carlos Alcaraz, who faces 18th seed Grigor Dimitrov in the last 16.
World number one Novak Djokovic is not in China.