Mikko Ilonen inflicted just a third ever defeat on Graeme McDowell at the World Match Play Championship in Kent.
Finland's Ilonen was four up after 10 holes but defending champion McDowell birdied the 11th, 13th and 15th to get back to one down with three to play.
However, McDowell missed a birdie putt on 17 and Ilonen holed out to win.
Scotland's Stephen Gallacher is out with one match remaining in the round-robin format after Pablo Larrazabal chipped in to win their contest on 18.
Spaniard Larrazabal hit his tee shot on the par-five last into a bunker and then blasted his second into deep rough. His third shot found similarly deep rough to the right of the green but he chipped in from 30 yards for a birdie.
It was a cruel way to go out for Gallacher, who missed a 15-foot birdie putt that would have halved the match.
"I was five under and have played two guys that haven't had any bogeys," said Gallacher who failed to win a point at the Ryder Cup last month and has lost his last 10 matchplay singles contests.
"It's not a jinx. What happened at the last is typical matchplay golf as he's won it with a birdie without having been on the fairway."
In the other match in Gabrielsson pool, France's Victor Dubuisson followed his opening-day win over Larrazabal with a 3&2 triumph over Ireland's Shane Lowry.
|World Match Play day two results|
|Victor Dubuisson (Fra) beat Shane Lowry (Ire) 3&2|
|Pablo Larrazabal (Spa) beat Stephen Gallacher (Sco) 1UP|
|Mikko Ilonen (Fin) beat Graeme McDowell (NI) 2&1|
|Joost Luiten (Ned) beat Alexander Levy (Fra) 4&3|
|Henrik Stenson (Swe) beat Francesco Molinari (Ita) 2&1|
|George Coetzee (SA) beat Thongchai Jaidee (Tha) 2&1|
|Jamie Donaldson (Wal) beat Jonas Blixt (Swe) 3&2|
|Patrick Reed (US) beat Paul Casey (Eng) 2&1|
Meanwhile, McDowell, who had won 15 of his 17 previous matches since making his debut in 2011, accepted that Ilonen had "putted better and that was the difference" after his rare defeat.
The Northern Irishman plays Joost Luiten, who beat Alexander Levy 4&3 to record his second win of the week, in Friday's final round of matches in Larson Group.
Sweden's Henrik Stenson was eight under over his last nine holes as he came from three down after four to beat Francesco Molinari 2&1.
South Africa's George Coetzee beat last year's runner-up Thongchai Jaidee 2&1 in the other match in the Palmer pool.
In the McCormack pool, all four players head into the final group match tied on two points each.
Welshman Jamie Donaldson recovered from losing to Paul Casey on Wednesday by beating Jonas Blixt 3&2, while Casey was defeated 2&1 by America's Patrick Reed.
The 16 competitors, divided into four groups of four, play three group matches each on Wednesday, Thursday and Friday before Saturday's quarter-finals and Sunday's semi-finals and final.