Rory McIlroy held a slender one hole lead at the midway stage of his WGC-Dell Match Play semi-final against Jason Day.
Both men had emerged from their quarters of the draw undefeated but not without problems, as Day was suffering slightly with a back injury and McIlroy had been taken to the 18th in three of his five matches.
The defending champion had extended his undefeated record to 12 at this event, though, and three birdies on the front nine moved him ahead of Day, who will return to Number One in the Official World Golf Ranking on Monday.
In the other semi-final, Louis Oosthuizen was two up against Rafa Cabrera Bello after eight holes.
Day got inside McIlroy's ball on the first and sank a 25-footer to move one up but a wayward second shot on the second resulted in a double-bogey and McIlroy made a birdie to level the match.
Both men parred the third in textbook fashion and McIlroy had a chance to move ahead on the par three fourth as Day missed the green but the Australian got up and down and McIlroy missed his birdie putt.
Pars followed on the fifth and the Northern Irishman made the green in two on the par five sixth to register a birdie and move ahead, but he missed the green on the par three next and the tie was all square once again.
McIlroy then put his approach on the eighth to ten feet for another birdie and took a one hole lead at the turn.
Cabrera Bello is the 52nd seed but has enjoyed a brilliant week at Austin Country Club, earning himself a debut at the Masters Tournament in April by virtue of his move into the world's top 50.
Oosthuizen has a 100 per cent record this week and overcame Jordan Spieth in the last 16 as he chases a first World Golf Championships win.
An 18-footer on the first moved Oosthuizen ahead before three shared pars, and a par on the fifth was enough to move the South African two up as both men failed to find the green in regulation.
Cabrera Bello recorded a double-bogey on the seventh after going from one side of the green to the other with his second shot to give Oosthuizen a three hole advantage, but the Spaniard clawed one back on the next as the 2010 Open Championship winner came up short with his second shot.
