Chapman finished the day at 14-under par. John Cook began the round tied with Chapman for the lead, but couldn't keep pace with the Englishman and settled for a 69 to drop into second place at Harbor Shores.
Chapman's 54-hole score of 199 tied the tournament record set by Sam Snead in 1973, although Snead was 17 under when he set the mark.
Steve Pate had a 67 to join 66-year-old Hale Irwin at 7 under. Irwin made a triple bogey on the par-3 fourth, but played well on the back nine and shot 69.
Chapman beat Padraig Harrington in a playoff to win a European Tour event in Brazil in 2000, and that's pretty much the highlight of his resume. He has never won on the Champions Tour. He made the cut in all 11 of his Champions Tour starts last year, but didn't have a top-10 finish.
He'll have one more challenge this weekend the final round Sunday, when the temperature is expected to reach 90 degrees.
The 53-year-old Chapman came to Harbor Shores after tying for 16th a couple weekends ago at the Mallorca Open — the European Senior Tour's lone event so far this season.
Chapman began to pull away from the field on the front nine Saturday. While Irwin was making a six on No. 4, Chapman rolled in his second birdie of the day to move to 9 under.
On the par-5 fifth, Chapman's second shot went into the rough to the left of the fairway. He calmly pitched out onto the green, giving himself a birdie putt pin high from about 25 feet, which he made.
Even his lone bogey was impressive in its own way. On the par-4 seventh, Chapman's drive went through the fairway, and he needed two hacks to get the ball out of some tall grass. But he was then able to pitch to within about 10 feet, and he made the putt to prevent further damage.
After making the turn at 9 under, Chapman birdied Nos. 10, 11 and 12 on his way to a 30 on the back nine. His iron play was superb, giving him birdie putts of 3 feet on No. 11 and about a foot on the par-4 14th. He made another 3-foot birdie putt on No. 16, then parred the last two holes.
Irwin shot a 66 on Friday to put himself in contention, and he would've had a similar round Saturday if not for trouble at No. 4. His tee shot missed to the left, where the green is protected by a wetland area. After going back to the drop zone, Irwin pitched onto the green, but then he three-putted from roughly the same distance as Chapman's birdie putt.
Play was delayed about three hours at the start of the third round on a wet morning. Warmer, more humid air is expected to hit Benton Harbor from the south Sunday.
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