While American world number two Jordan Spieth struggled for a second week in a row, Day put himself in good position to vie for a fourth PGA Tour win in five starts by carding a three-under-par 68 on a tricky day for scoring at the TPC Boston.
The 27-year-old Australian, who won the opener in the lucrative four-event playoff series by six shots at The Barclays last week, birdied five of his first 13 holes to set the early tournament pace before slipping back with two late bogeys.
“I was making everything I looked at, and then it slowly dried up,” world number three Day told reporters about his putting on lightning-fast greens in breezy conditions.
“It was a very patient day. Once you’re frustrated, you’re going to make some mental errors and it’s going to be bad for you.”
Burly Zimbabwean de Jonge, still seeking a first PGA Tour win in his eighth full-time season on the U.S. circuit, was delighted with his round after recent struggles and missed cuts in his previous two starts.
“I’m more shocked than anything else,” the 35-year-old said after racking up seven birdies and a lone bogey in his 65 to seize a two-stroke lead.
“It’s been a rough stretch of golf for the last couple of months so to shoot something like this was a pleasant surprise.”
England’s former world number one Luke Donald, precariously placed at 87th in the FedExCup standings coming into this week, was among a group of nine players tied for second after opening with a 67.
Masters and U.S. Open champion Spieth, who missed the cut at The Barclays last week after struggling with his driving and wedge play, endured further problems with his short game as he piled up eight bogeys and four birdies in an opening 75.
That left him a distant 10 strokes off the pace and the 22-year-old Texan, clearly frustrated, declined to speak to the media after signing his card.
Northern Irish world number one Rory McIlroy, back on the PGA Tour after skipping The Barclays last week, got to three under before bogeying his last two holes for a 70.
“I felt like I hit the ball very well,” said McIlroy. “These greens are probably the fastest I have putted on in a while.
“I was hitting the ball a little bit too hard at times and under-reading them … I just need to adjust to that.”
The top 70 players on the FedExCup points list after the Deutsche Bank advance to the Sept. 17-20 BMW Championship in Lake Forest, Illinois where the leading 30 will qualify for the Tour Championship finale in Atlanta the following week.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue/Larry Fine)