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.
"I don't think my scoring was great all the way through to be honest.
"This is going to hurt and I will bounce back."
Humphries will play number 10 seed Michael Smith in the last eight and says Cross 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.
"It's all about the fightback. I'm hugely proud of myself.
"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.
His rival took the match into a decider and won the final set 3-1.
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.