Five-time world champion Ronnie O'Sullivan was beaten 6-5 by Mark Williams in the quarter-finals of the International Championship in China.
Despite losing the first three frames, the Welshman claimed his first ranking event win over O'Sullivan since 2002.
Northern Irishman Mark Allen beat Wales' Michael White 6-3 to book his place in the semi-finals.
England's Robert Milkins saw off Marco Fu 6-5 and compatriot Ricky Walden was a 6-1 winner over Scot Jamie Burnett.
Englishman O'Sullivan, 38, who compiled three century breaks in his 6-1 second-round victory over Anthony McGill, managed a best of 72 against Williams.
O'Sullivan started strongly before the Welshman, 39, opened up a 5-3 lead, only to allow O'Sullivan to level at 5-5.
O'Sullivan potted only two reds in the deciding frame as Williams, world champion in 2000 and 2003, made breaks of 50 and 22 to maintain his hopes of a first ranking title since the 2011 German Masters.
"I haven't beaten him for so long, eventually I've got rid of that pain," Williams told worldsnooker.com.
"I was shocking for the first few frames and dragged Ronnie down, but after the interval I was a different player.
"It's an excellent win for me, even though Ronnie wasn't at his best. He's still the best player in the world."
O'Sullivan, who is yet to win a title this season, said: "We both started poorly and I was struggling with my concentration. Then he played well to go 5-3. He's in good touch. At 5-3 I felt I had to make something happen, and that was the best I felt in the match."
International Championship quarter-final results:
Ricky Walden (Eng) 6-1 Jamie Burnett (Sco)
Robert Milkins (Eng) 6-5 Marco Fu (HK)
Mark Allen (NI) 6-3 Michael White (Wal)
Mark Williams (Wal) 6-5 Ronnie O'Sullivan (Eng)