Lewis carded an even-par 71 in the third round to hold a share of the lead entering the final round. Lewis was tied at 12-under-par 201 with Chris Riley, who notched a 5-under 66 in the third round.
Singh, who posted a 2-under 69 in the third round, was 2 under through his first five holes of the final round to join Lewis atop the leaderboard. Lewis, who is paired with Singh and Jonathan Byrd, was even par through five holes.
Heavy rain forced Saturday's third round to be washed out. The original plan of playing 36 holes Sunday was done in when the TPC at Deere Run remained unplayable from the rain.
Course Superintendent Chris Hague said the course took on 'close to 3.2 inches of rain in a 48-hour span.'
Third round play finally began at 12:00 PM ET on Sunday. After the third round was completed, final round play began shortly there after.
Notah Begay III is 1 under through seven holes and shares third place with two other players. Chris Riley is even par through five holes, while Paul Stankowski is 1 under through five holes to share third place at minus-11 with Begay.
Lewis stumbled out of the gate during the third round. He bogeyed the first and fourth, but did manage to notch a birdie at the second. He climbed back to even par for his round with a birdie at the par-5 10th.
The 1999 John Deere Classic winner stumbled to another bogey at the 15th, but managed to grab a share of the third-round lead with a birdie on No. 17.
Lewis had a steady start to his final round. He opened with five consecutive pars to remain atop the leaderboard.
Singh began his third round with a birdie at the second, but settled in to reel off 10 straight pars. He then notched back-to-back birdies from the 14th to move to minus-11. However, he stumbled to a bogey at the 18th.
The Fijian birdied the second and fourth in the final round to join Lewis in first place.
Riley posted a bogey-free 66, including four birdies on the back nine, to tie Lewis for the lead after the third round. He fell into a share of third when he bogeyed the par-5 second.
Byrd stands alone in sixth place at 10 under par through five holes of the final round. He had carded a 1-over 72 in the third round.
Tom Gillis has moved up the leaderboard early in the final round. He is 4 under through 12 holes and stands at 9 under par. He is joined there by Hidemichi Tanaka, even par through seven holes, and Glen Day, 3 under through nine holes.