Spieth beat Tom Gillis on the second hole of a playoff to win at TPC Deere Run on Sunday for his fourth victory this season.
Gillis and Spieth, who opened the tournament with an even-par 71, finished the final round at 20-under 264.
"To be able to shoot 20-under in three rounds is nice momentum," Spieth said. "The finish, when I really didn't feel like tee to green I had much [Sunday], gives me a lot of momentum to draw on if I don't have my best stuff. "
Gillis, who at 46 was trying to become the oldest first-time winner on the
Spieth heads to the British Open in search of his third consecutive major championship.
He is the first golfer to win four times before the Open Championship since Tiger Woods in 2000, and he silenced critics who questioned why he played this tournament instead of heading overseas early to prepare for St. Andrews.
"I really didn't care anyways. I came here for a reason, and we accomplished that reason," Spieth said.
After shooting a career-best 10 under Saturday, Spieth began the final round with a two-shot lead.
But the field caught up to Spieth when he turned in a pedestrian front nine.
Spieth bogeyed two of his first three holes and was soon passed by the likes of Gillis and Johnson. Spieth was even at the turn and two shots back of Gillis, who in 171 previous Tour starts had finished in the top three only twice.
Spieth's low point was a bogey on the par-four 11th hole after a putt from the fringe went left.
But Gillis played his final six holes at even par. Spieth rallied back into contention with four birdies in five holes, including a 21-foot chip from the fringe on No. 16.
Spieth had a 30-foot putt for the win on No. 18, but he pushed it left. He nearly won on the first hole of the playoff, but his 33-foot try lipped out.
Spieth said he has work to do ahead of the British Open.
"I need to fine tune my driver. I love where my putter is at. My pace control, my speed is awesome right now," Spieth said.
Gillis matched Spieth with a par on the first hole in sudden death. But a tee shot that went into the rough led to a shot in the water, effectively ending his bid for a victory.
Gillis shot a seven-under 64 in the final round.