Louis Oosthuizen held a one-hole lead at the midway point of his winner-takes-all Group 16 match against Andy Sullivan at the WGC-Dell Match Play.
Both players came into the final day of the group stages with a 100 per cent record, knowing that a victory would see them advance to the last 16 at Austin Country Club.
The duo were the top two seeds in their group and that class showed with just two bogeys and a concession between them in the first nine holes.
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 in front and the pair remained separated by one at the turn.
Bernd Wiesberger was one up through eight holes against Matt Jones in the other Group 16 match.
Patrick Reed was also involved in a winner-takes-all clash and he was threatening to blow Phil Mickelson away in Group 9 as he went six up after 11 holes.
Pars were enough to win the first two holes and Reed then went birdie-birdie-eagle from the fourth to take control. He parred the ninth and then holed out from from 95 yards for another eagle on the next, before Mickelson grabbed a hole back.
Matthew Fitzpatrick was handed victory in his Group 9 match after Daniel Berger withdrew with a wrist injury sustained on Thursday.
World Number One Jordan Spieth was on course to advance from Group 1 at three up through seven holes against close friend Justin Thomas, while Jamie Donaldson was one up against Victor Dubuisson.
In Group 8, where each player had one point, Kiradech Aphibarnrat was all square against Robert Streb through ten holes, with Dustin Johnson three up on Jimmy Walker.