Taiwan's Chuang Chih-Yuan felt like he had to "kill his brother" after overcoming best friend Adrian Crisan at the London 2012 Olympics.
Chuang and Romanian Crisan share a bond off the table, but met on it for a place in the last four of the men's singles at the ExCeL Arena.
Despite the attachment, Chuang was ruthless as he dominated Crisan, who pulled off a shock yesterday night by knocking out fourth seed Timo Boll.
Indeed, Chuang was in charge throughout, wrapping up a straight-games victory in just 20 minutes.
"He is my best friend," said Chuang afterwards. "I was sorry when I played him; it is like you are in a war and have to kill your brother."
Germany's Dimitrij Ovtcharov and Denmark's Michael Maze then battled out a seven-game thriller.
The former looked comfortable in moving into a 2-0 lead, with his opponent being yellow carded at one stage for showing his frustration.
However, Maze - who last night beat third seed Jun Mizutani - composed himself and, playing with extra power, reaped rewards from forcing the German on to his backhand to level matters.
They then took a game apiece, after which Ovtcharov edged the decider 11-9, much to the dismay of Maze's legion of fans.
Half of the quarter-final games were completed, with Chinese stars Zhang Jike and Wang Hao in action tomorrow morning.
The former will tackle Hong Kong's Jiang Tianyi for a chance to face Ovtcharov in the last four, while the latter will meet Chuang should he overcome Seiya Kishikawa.
