Walking past the women's toilet at the five-star hotel, the couple claimed to have heard screams coming from inside.
They were surprised to find a man inside.
The woman's husband asked the man what he was doing there and told him to leave.
But the man allegedly attacked him, raining blows on his face until he became unconscious. He later needed 20 stitches on his face.
Singaporean Elaine Ong, in her 40s, and her husband, Mr Robert Pferdmenges, 64, a German national, have been staying in the Grand Hyatt Singapore along Scotts Road.
They said they have stayed in the hotel's Grand Club deluxe room for the last six years under a long-term arrangement costing them $400 a night.
They told The New Paper that they were on the 21st floor and about to walk past the women's toilet with its main door ajar when they heard a scream on Sunday night.
Mr Pferdmenges said a man and a woman, both dressed, were inside.
He said: "I asked the man, 'What are you doing inside a female toilet? Get out.' That's when the man punched me.
"Within five seconds, I was on the floor, passed out. When I came to, I was on a stretcher on the way to hospital."
Ms Ong claimed she was attacked by the woman.