LOS ANGELES (AFP) - Australia's Jason Day became the first player to book his place in the knockout stage of the WGC Match Play on Friday after England's Paul Casey withdrew from their final group stage match.

Casey was forced to concede after only six holes at the Austin Country Club after complaining of stomach cramps.

Casey's withdrawal ensured Day qualified as winner of Group 2 following earlier wins over Thailand's Thongchai Jaidee and Northern Ireland's Graeme McDowell.

Elsewhere Friday, 14th seed Zach Johnson of the United States secured his place in the weekend field with a comfortable 4 and 3 defeat of Ireland's Shane Lowry.

Johnson's passage to the knockout rounds as Group 3 winner never looked in question as he raced into a commanding lead over the front nine.

He reached the turn at four up before a birdie on the 393-yard par four 10th took him to five up.

Johnson will play Bill Haas on Saturday. Haas became the third player through to the next round after securing a one up victory over Australia's Adam Scott.

Haas rolled in a 12-foot birdie putt on the 18th to secure a win which saw him top Group 6 following wins over Belgium's Thomas Pieters and Chris Wood of England.

Haas however was unhappy with his long game and believes world number 15 Johnson will start as favorite for their last 16 clash on Saturday.

"I struggled all day - I need to play better tomorrow if I want to advance," Haas said. "You never know in match play because you've only got to beat one guy.

"But the way he's playing I don't think I have a chance. But he may have a down day and I get hot tomorrow," said Haas, who nevertheless drew encouragement from his impressive putting game.

"That's the only reason I'm here - because of the greens," Haas said.