Bruce Lee endured "two days of hell" when he sprained his ankle badly while landing awkwardly from a jump from a high jump on a slipped mattress, and had to be driven to Bangkok to see a doctor, where he caught a virus in the hot and stuffy conditions. Close-ups were used to finish the fight, as Bruce struggled and had to drag his leg, which was covered up by, and contributed to, his character's worn out, exhausted appearance. He couldn't move properly and was also racked with aches and fever and was having difficulty keeping food down. Even so, filming continued. His twisted ankle meant that he had to drag his injured leg, so in several scenes he had to be filmed in closeup. He also broke a glass in his hand, resulting in a gash that required ten stitches. While at the hospital in Bangkok, he caught flu and rapidly lost ten pounds.
In the first week of filming, Bruce Lee was increasingly annoyed by the haphazard production. The equipment used for filming was old and in bad repair and the script consisted of a few basic ideas scribbled on scraps of paper.
The Thai brothel featured in the film was actually a genuine and functioning brothel. The extras who feature in these scenes (excluding Malalene's character) were actual prostitutes who were paid one to two hundred Baht each by Golden Harvest, more than they would normally receive in a day by their clients, so that they could appear in the film. The actual bedroom scenes however were filmed in a riverside bungalow owned by the nearby New Wan Chai Hotel (now the Rimtarninn), where the film crew stayed during filming, due to the bedrooms in the brothel being smelly and unhygienic.
In 1971, this was the highest grossing film of all time in Hong Kong, beating out American productions La novicia rebelde (1965) and ¡Tora! ¡Tora! ¡Tora! (1970).
If you pay attention through the movie, you can tell when or not Cheng will kill in a fight. When it's just a "fun" fight, he would wear either brown or blue pants with a t-shirt and a blue sash. But in a serious fight, he'd wear a long sleeve shirt with black pants and a white sash. This may be because in Chinese culture, white is a symbol of death.