Before today, all 14 completed matches had gone in favour of the seeded players, but the unprecedented scenario of all 16 seeds reaching the second round at the Crucible Theatre was dashed after Si Jiahui lost 3-10 to Hossein Vafei.
The 'Prince of Persia' put together a remarkable run of nine consecutive frames.
Vafaei had trailed 1-3 in the opening session on Wednesday, but turned the match around to lead 6-3 overnight. Si started today's session better, but missed a pink to the middle on 22 points, allowing his opponent to clear up with a 53 for 7-3.
In the 12th frame, Si was on nine points when he lost position, after which Vafaei won a safety battle and added 45 points to extend his lead.
Hossein Vafei fighting for Iran
With a break of 78, the Iranian - who has struggled to focus on the sport due to the political situation in his homeland - put himself firmly on course for victory.
Si had one last chance to mount a comeback, but missed a red on 24 points in the 12th frame, and Vafei finished the match with a 68.
"I get a lot of support from home, so I fight for them," said Vafei. "I just want to make them happy." He hopes the war, which began on February 28th with US and Israeli attacks on Iran, will soon be over. "Then safety will return to my country."
Shaun Murphy and Xiao Guodong into the last 16
On the other table, 2005 Crucible champion Shaun Murphy built a comfortable 6-2 lead after the first session of his second-round match against Xiao Guodong.
Murphy, who only just scraped through the first round against Fan Zhengyi, was sharp from the start and at times played inspired snooker - even though in the third frame, after snookering himself, it took him an incredible eight attempts to escape.
Nevertheless, he won the first five frames, and despite Murphy's snooker troubles, Xiao never really had a chance. Murphy produced breaks of 79, 103, 63 and 64 along the way.
The Chinese player didn't give up, reducing the deficit to 2-5, though without making a significant break. Murphy then closed out the session with another break of at least 50 (61), taking a deserved 6-2 lead into tomorrow's second session.