The tournament serves as a tune-up for next week's Senior PGA Championship in Akron, Ohio and features senior golfers sharing the Upper Montclair Country Club course with NFL players past and present.
NFL players likely were impressed with the threshold of pain demonstrated by Eichelberger, who had eight birdies and two bogeys.
"I couldn't get out of bed on Wednesday morning. The back seized up on me," said Eichelberger, who birdied the 13th, 14th and 15th holes before parring the last three. "I was able to relax enough today to hit a few balls. The therapists said it would get a little better each day."
Eichelberger is 20th on the money list and posted his lone win of the season -- and sixth of his career -- at the Emerald Coast Classic.
"It was 100-to-1 to be here in the interview room when I started this morning," Eichelberger said. "I didn't make any adjustments. I initially thought I'd need a more erect stance, but I didn't."
Also fighting long odds is Mason, a teaching pro from Dillard, Ga., who needed a 5-under 67 in qualifying on Tuesday to make his way into the field. Mason is playing just his seventh Senior Tour event and second this season.
Mason, who was 57th at last week's Farmers Charity Classic, had eight birdies, including 20-foot putts at Nos. 5, 7 and 8.
"I shot a 68 last week, so maybe today was a nice springboard for the rest of the year," said Mason, who credited help from a local caddie for making it easier for him to read his putts. "I was really calm today. I had been real uptight in these events."
Dressed in knickers he has work for two decades, Mason finished strongly with putts on 16 and 17 and a par at the 18th hole. He hit 15 of 18 greens.
John Bland is two strokes back after shooting a 67 while Dave Barr and Jay Sigel are three shots off the pace after firing 68s.
Sigel ended an injury-plagued four-year victory drought Sunday with a two-stroke victory at the Farmers Charity Classic.
Defending champion John Schroeder carded a 73.
Senior Tour money leader Hale Irwin, who turns 57 on Monday and received an exemption to the U.S. Open in two weeks, is not entered.
NFL players compete in a separate competition that began Thursday.
Former Tennessee Titans kicker Al Del Greco, who seeks his sixth title, shot a 73 Friday and shared the lead with former Miami Dolphins defensive back Dick Anderson.