Louis Oosthuizen overcame fellow European Tour Member Andy Sullivan to maintain his 100 per cent record and advance to the last 16 at the WGC-Dell Match Play.
Both players came into the final day of the group stages with two wins from two, knowing that a victory would see them go through at Austin Country Club.
Oosthuizen has made the quarter-finals at this event in the past two seasons and the World Number 18 just edged out Sullivan, who was second seed behind him in the group.
A birdie on the second moved Sullivan ahead but the Englishman was forced to concede the next after getting in trouble off the tee. A Sullivan bogey on the fifth moved Oosthuizen ahead and the pair remained separated by one at the turn.
A concession from Oosthuizen squared things up on the tenth but the 2010 Open Championship winner then claimed the 12th, 14th, 15th and 16th to win 4 and 2.
"I played really solid," he said. "I didn't make a lot of birdies. I had a lot of putts shave the edges. But I gave myself really good opportunities and I put pressure the whole time on Andy coming in. It was just in my favour today."
Oosthuizen will take on Jordan Spieth in the last 16 after the World Number One also held on to his perfect record with a 3 and 2 victory over fellow American and close friend Justin Thomas.
"I will stick to what I've been doing around this golf course," he added.
I'm playing the guy. If he makes a mistake, you are going to play the hole a little differently. It's going to be a big task, but I feel like I'm up to it - Louis Oosthuizen
Patrick Reed was also involved in a winner-takes-all clash and cruised past Phil Mickelson 5 and 4 in Group 9.
Pars were enough to win the first two holes and Reed then went birdie-birdie-eagle from the fourth to take control, with the help of a 43-foot putt on the sixth. He parred the ninth and then holed out from from 95 yards for another eagle on the next to go seven up before Mickelson grabbed holes back on the 11th and 13th.
"I love match play," he said. "I relish every time I get to go in this format and go up against one guy.
It's something that I love and something I'm pretty good at. Just going out and getting in a dog fight - Patrick Reed
Matthew Fitzpatrick was handed victory in his Group 9 match after Daniel Berger withdrew with a wrist injury sustained on Thursday.
Reed will next face Dustin Johnson, who beat Kiradech Aphibarnrat on the first play-off hole to move through from Group 8.
Johnson had earlier beaten Jimmy Walker 2 and 1, while Aphibarnrat overcame Robert Streb by one hole.Bernd Wiesberger and Matt Jones halved their dead rubber in Group 16, with Jamie Donaldson beating Victor Dubuisson by one hole in Group 1.