Knost has twice lost his spot on the tour and had to return to the PGA Qualifying tournament last year to regain his eligibility.
His 66 gave him a 36-hole total of 9-under 133 at the Harbour Town Golf Links. Carl Pettersson shot the low round of the tournament, a 65, to finish alone in second at 135.
It took a score of 3-over 145 to make the cut. Among those who survived was John Daly, making his second appearance on the tour this year.
Daly managed a 74 for a 144 total. World No. 1 Luke Donald was also at 144.
Among the big movers in the second round was Jason Dufner, who was a co-leader at the Masters after 36 holes last week. Dufner shot a 66 and climbed 67 places into a tie for 41st.
Matt Kuchar, who finished tied for third at the Masters, was at 141 after 36 holes.
Masters champion Bubba Watson did not play in this event.