Brothers Deane and Brenden Pappas posted matching rounds of 6-under 66 to join Loren Roberts, Bill Glasson, Patrick Sheehan and David Sutherland in a tie for fourth.
Frazar kicked off his round with a birdie at the par-4 third and kept on rolling with an eagle at the par-5 fifth. He then birdied the seventh and added a birdie at the par-4 ninth to go out in 31.
The 33-year-old continued his fine play on the inward half with a birdie at the par-5 11th, but he found trouble with a bogey at the very next hole.
Frazar, who finished in the top-10 in three of his first four events this year, parred his next four holes before a birdie at the par-4 17th lifted him into the group at minus-6. Frazar then secured a share of the lead with a birdie at the par-5 18th.
'I'm not paying too much attention to scoreboards right now,' said Frazar. 'The greens were nice and soft. They rolled really well early on, but as the day went on they got a little bumpy.'
Day ran off four birdies on the front side and collected back-to-back birdies starting at the par-5 11th to move to minus-6. Day then birdied the par-5 18th to join Frazar in the lead.
Senden struggled early with a double bogey at the second, but he birdied five of his next seven holes to make the turn at 3 under. The Australian then picked up a remarkable eagle at the par-4 10th, but gave a shot back with a bogey at the very next hole.
The 33-year-old got closer to the leaders with a birdie at the 16th and capped off his round with an eagle at the closing hole to join Frazar and Day atop the leaderboard.
Brenden Pappas, the runner-up last year, collected three birdies and two bogeys over his first nine holes at Annandale Golf Club. The 34-year-old picked up steam on the inward half starting with a birdie at the par-5 11th and back-to-back birdies from the par-4 13th.
The South African added a birdie at the 16th and closed with a birdie at the last to finish one shot off the lead.
Deane Pappas, who finished second at this event two years ago, played the back side first and picked up his first birdie of the day at the 13th. He stumbled to a bogey at the very next hole, but recovered with two straight birdies starting at the par-3 15th.
The 36-year-old birdied the 18th and added a birdie at the par-4 first to make to reach 4 under. He made it three straight with a birdie at the second and joined his brother in a tie for second with a birdie at the fifth.
Roberts, an eight-time winner on the PGA Tour, ran off three birdies on the front side and added back-to-back birdies from the 10th to jump to 5 under.
The 49-year-old faltered with a bogey at the 14th, but countered with birdies on each of the next two holes.
Glasson, whose last win on the PGA Tour came at the Las Vegas Invitational in 1997, started at the 10th and mixed seven birdies with a bogey for his 66. Sutherland used a strong finish with three birdies and an eagle over his last seven holes for his share of second.
Sheehan birdied each of his last three holes on Thursday to finish within one of the lead.
Billy Andrade, Steve Pate, Carl Pettersson, Craig Barlow and Chris Smith are knotted at 5-under-par 67.
Defending champion John Huston finished in a group at 3-under-par 69.