Northern Ireland's Rory McIlroy can end the week as world number one by winning the WGC Match Play in Arizona.
Second seed McIlroy came from behind at the turn to while current world number one Luke Donald lost to Ernie Els.
Donald's first-round defeat means McIlroy or Lee Westwood will take over at the top if they win this week.
"It is just nice to get through," said McIlroy. "It is another [thing] knowing you could be world number one."
The world number two added: "That stretch [on the back nine] where I won four in a row felt like I played some good golf."
McIlroy, who plays Denmark's Anders Hansen in the last 32 on Thursday, is the only Northern Ireland player left in the tournament following defeats to Graeme McDowell and Darren Clarke.
McDowell was beaten 2&1 by the South Korean YE Yang, and reigning Open champion Clarke lost by 5&4 to the USA's Nick Watney.
McDowell said: "I made six birdies and played decent golf, but I ran into a man who played extremely well.
"He did not put a foot wrong, but that is just the way the cookie crumbles with this tournament.
"He seems to have my number a little bit - and I have a tendency to bring out the best in guys around this track for some reason."