Lurz left it desperately late before overhauling Greek rival Spyros Gianniotis in the final metres of the gruelling event for his third medal in three races at Jinshan City Beach.
"I was sure that I won only when I touched the finish board, the slap board. Then I already knew that I won," Lurz said.
"But before the race I didn't think about winning five times or six times or seven times in a row. I think this is big pressure. Every time I go on the start, I try to do my best and don't think about the number of titles."
Gianniotis, who enjoyed a rare victory over defending champion Lurz in the 10km, thought he had the race won as he powered towards the finish and was stunned when he finished in second place.
"The last few hundred metres, I was in the lead and I knew Thomas was behind," Gianniotis said. "He began to fight back, and I just pushed ahead, thinking I was ahead and I had nothing or no one to lose. It's very, very hard."
Bronze medallist Evgeny Drattsev of Russia said Lurz, who also won the event in 2005, 2007 and 2009 – when he also won the 10km – had proved himself as one of the world's great athletes.
The 31-year-old Lurz placed second in this week's 10km and helped Germany win bronze in the team 5km event, swelling a bulging trophy cabinet which also contains numerous other international titles.
"Lurz is a very strong swimmer. He is a great athlete in history," Drattsev said.
In the women's event, Switzerland's Swann Oberson took gold with Aurelie Muller claiming France's first medal of the world championships and American Ashley Twichell taking bronze.
"I swim five hours a day. The swimming pool is my second house," Muller said.
Meanwhile Chinese star Wu Minxia topped the semi-finals of the women's 3 metre springboard as the hosts seek an unprecedented sweep of all 10 diving medals.
China are expected to win their eighth title in the men's 3 metre final later.
US star Michael Phelps and Brazilian champion Cesar Cielo, who avoided a doping ban after testing positive for a banned diuretic, both trained at Shanghai's Sea Crown pool Friday before the swimming events start Sunday.