Dallas - James Hahn fired his second bogey-free 64 of the week on Saturday while Australia's Jason Day charged into contention entering the final round of the PGA Tour's Byron Nelson tournament.
World number three Day blasted a seven-under par 63 in the third round at the TPC Las Colinas course just outside Dallas, but was two strokes off the pace as Hahn grabbed the lead on 12-under 198.
"I played well out there. Overall it was a good day," said Hahn, the 18-hole co-leader. "It is really close. One putt here or there. You need good breaks to happen to keep the momentum going and I did that today."
Fellow American Billy Horschel was second on 199 after a 66 that included birdies on his last three holes.
Hahn said he won't worry while sleeping on the lead, even though there were a dozen players within five strokes.
"Tell me what my tee time is tomorrow and I'll just show up," he said. "There's a lot on the line but I feel like when I'm on the golf course there's a different animal that comes out and I get very competitive."
Day had eight birdies to move into a share of third with second-round leader Jason Kokrak, who shot 72. American Cameron Tringale was fifth at 201 after a 67.
Day made an incredible 60-foot birdie putt on the par-3 17th to move into a share of the lead at one stage.
Reigning Masters champion Sergio Garcia of Spain fired a 64 to finish on 202 and share sixth with former Byron Nelson champion Jason Dufner (65) and Bud Cauley (68).
Defending champion Garcia shot 29 on the back nine Saturday, the same score he posted on the front nine in the opening round when he won in 2016.
Americans Scott Brown (65), Scott Piercy (66), Grayson Murray (68), Chad Campbell (68) and South Korea's An Byeong-Hun (69) were on 203.
World number one Dustin Johnson shared 19th after carding a one-over 71 Saturday, snapping a streak of 14 consecutive par-or-better rounds at the Byron Nelson for Johnson, who is seven shots behind Hahn.
"Mentally and physically I'm fine," Johnson said. "I'm not as sharp as I was, obviously, before the Masters and it's going to take a little time to get back in that kind of rhythm but I'm seeing good signs."
Leading scores after the third round of the PGA Tour's Byron Nelson tournament at Irving, Texas (par-70, USA unless noted):
198 - James Hahn 64-70-64
199 - Billy Horschel 68-65-66
200 - Jason Day (AUS) 68-69-63, Jason Kokrak 66-62-72
201 - Cameron Tringale 66-68-67
202 - Sergio Garcia (ESP) 73-65-64, Jason Dufner 69-68-65, Bud Cauley 67-67-68
203 - Scott Brown 71-67-65, Scott Piercy 68-69-66, Grayson Murray 72-63-68, Chad Campbell 68-67-68, An Byeong-Hun (KOR) 68-66-69
204 - Kyle Reifers 69-69-66, Scott Stallings 72-66-66, Matt Kuchar 66-71-67, Danny Lee (NZL) 70-70-64, Sean O'Hair 67-69-68
205 - Louis Oosthuizen (RSA) 69-70-66, Tony Finau 75-65-65, Ryan Armour 67-69-69, Kevin Tway 71-70-64, Dustin Johnson 67-67-71
206 - Joel Dahmen 68-70-68, Jamie Lovemark 72-66-68, Gary Woodland 70-68-68, Kang Sung (KOR) 69-68-69, Brooks Koepka 67-69-70, JJ Henry 68-69-69