Mickelson finished group play in style at 3-0-0 on Friday with a 6&5 thumping of fellow American J.B. Holmes at Austin Country Club.
Leishman also advanced to the knockout phase of the World Golf Championship event, but only barely, winning a three-way playoff with Englishman Lee Westwood and Pat Perez after they all finished with the same 2-1-0 record in their group.
"(I'm) having a good week or so," Australian Leishman, who won the Arnold Palmer Invitational last Sunday, told Golf Channel.
Asked for his thoughts about facing Mickelson, known for his light-hearted trash talking, the laid-back Leishman sounded unfazed.
"I'll have to work on my trash talking I guess," Leishman said. "I've got to keep hitting it the way I have been and make some putts."
Five-times major champion Mickelson found himself in an unfamiliar position.
"I'm excited. I haven't been to the weekend in match play in a long time," he said.
Among other round-of-16 match-ups will be Spaniard Jon Rahm versus American Charles Howell III, Englishman Paul Casey against Japan's Hideto Tanihara and Dane Soren Kjeldsen versus American William McGirt.
World number one Dustin Johnson also advanced, finishing group play 3-0-0 with a 5&3 win over American compatriot Jimmy Walker.
"I've played really solid every day," Johnson said. "Today I made a bunch of birdies, no bogeys and usually in match play you do that, you win.
"Just because I'm number one I don't get a pass. I've still got to play really well, work hard. Being number one makes me want to work a little harder so I can stay there."
Rahm won his group by beating countryman Sergio Garcia 6&4.
"I'd rather not play a fellow Spaniard but luckily whoever won today was going to make it through," Rahm said.
Howell won his group after an eventful three-way playoff with Englishman Tyrrell Hatton and Spain's Rafa Cabrera-Bello.
Hatton was eliminated at the first extra hole after incurring a two-stroke penalty for putting without replacing his ball after he had caused it to move fractionally. Howell finally emerged victorious four holes later. (Reporting by Andrew Both in Cary, North Carolina; Editing by Larry Fine)
Read the original article on Eurosport: Mickelson into knockout stage at Match Play