Louis Oosthuizen held a two hole lead at the turn as he took on World Number One Jordan Spieth in the last 16 at the WGC-Dell Match Play.
Both men held a 100 per cent record from their three matches in the group stages, with Oosthuizen having made the quarter-finals at this event in the last two seasons.
He was on course to make the last eight again at Austin Country Club, with two birdies enough to put him ahead at the turn.
The first three holes were halved but when Spieth hit his tee shot on the par three fourth right, Oosthuizen took advantage and moved ahead with a two-putt par.
Spieth missed the green again on the next par three at the seventh, this time coming up short, and the South African was again on hand to capitalise, putting his tee-shot to six feet for a birdie.
The American played the eighth beautifully, finding the middle of the fairway with his drive and then putting his second to 12 feet for a birdie to close the gap.
But he found a bunker off the tee on the ninth and a par was enough to give Oosthuizen a two hole cushion.
Patrick Reed was one down to Dustin Johnson in their round of 16 match-up.
Reed took the lead with a birdie on the par five sixth but a 41-foot putt from Johnson on the seventh levelled matters before bunker trouble off the tee saw Reed drop the ninth.
