SILVIS, Ill. -- Jordan Spieth made it a lot tougher on himself than it needed to be.
In the end, Spieth got exactly what he was looking for from the John Deere Classic: a win and some positive energy ahead of The Open.
Gillis and Spieth, who opened the tournament with an even-par 71, finished the final round at 20-under 264. Zach Johnson and Danny Lee tied for third at 19 under.
"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. "
The 46-year-old Gillis, who was attempting to become the oldest first-time winner on the PGA Tour in 20 years, sent an approach into the water. Spieth made par for his second win at TPC Deere Run in three years and improved to 3-2 in playoffs in his career.
Now Spieth heads to The Open in search of his third consecutive major championship. Gillis' consolation is a bid into The Open. It will be his first major appearance since the 2011 PGA Championship.
Spieth is the first golfer to win four times before The Open since Tiger Woods in 2000 -- and he silenced the 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.
The fifth PGA Tour win of Spieth's career comes two weeks before his 22nd birthday. Only Woods (6) has more tour wins before turning 22 since 1940.
After shooting a career-best 10 under Saturday, Spieth entered the final round with a two-shot lead. But the field caught up to him 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 PGA Tour starts had finished in the top three twice.
But Gillis played his final six holes at even par. Spieth roared 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.
Even though Spieth ultimately outlasted the field, he knows he has some work to do -- specifically with his driver, which abandoned him at times this weekend -- if he hopes to make it three majors in a row.
"I need to fine-tune my driver," Spieth said. "I love where my putter is at. My pace control, my speed is awesome right now."
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 an elusive victory.
"When you're going to be 47, the window is closing," said Gillis, who shot a 7-under 64 in the final round. "What I saw [Sunday] and the last three days, I'd have to say makes me think I've still got some tread on the tires left."
Lee, who was attempting to become the first player in 18 years to get his first two career victories in back-to-back events, bogeyed No. 18 when par would have earned him a playoff spot.
Johnson's round was delayed when, while standing over a possible birdie putt on No. 16, a loud booming sound that apparently came from a boat on the nearby Rock River startled him. He missed the subsequent putt.
The Associated Press and ESPN Stats & Information contributed to this report.