Defending champion Rob Cross was knocked out of the PDC World Championship after suffering a shock 4-2 defeat against Luke Humphries.
Second seed Cross, 28, looked in control at 2-0 up but his 90th-ranked opponent found his range on the doubles and rattled off three successive sets.
Humphries, 23, had the darts at 2-2 in the sixth set and missed one dart at double seven to take the match.
Cross failed to finish and Humphries returned to book a quarter-final spot.
"I'm disappointed," Cross said. "I didn't feel like I played. I felt a bit fatigued. Fair play to Luke, he played well.
Humphries will play number 10 seed Michael Smith in the last eight and says Cross, who became the first player to win the competition on his first attempt in January, serves as an inspiration.
"I felt like I should have been 1-1 but I said to myself, 'If you want to be world champion you cannot give up'," said Humphries.
"People don't think I can win this but I take inspiration from Rob - I can win this."
In the evening's other matches, Dave Chisnall set up a quarter-final against Gary Anderson after a comfortable 4-0 win over Welshman Jamie Lewis, while Smith beat Ryan Searle 4-1.
Earlier on, James Wade missed five darts to reach the quarter-finals before being knocked out 4-3 by world number 74 Ryan Joyce.
The ninth seed, who had apologised for his behaviour during and after victory over Seigo Asada in an earlier round, led 3-2 in sets, only for his finishing to desert him when he needed it most.
Joyce will play world number one Michael van Gerwen in the last eight.
Northern Irishman Brendan Dolan is also through to the quarter-finals in London for the first time after a 4-1 victory over Dutchman Benito van de Pas.
England's Nathan Aspinall progressed with a 4-3 win over South African Devon Petersen.