Greg Norman shot a 3-under 67 to be three shots off the pace at the Senior Players Championship in Pittsburgh, his first Champions Tour event since 2009.
Bruce Vaughan shot a 6-under 64 to take a one-stroke lead over Fred Funk in the third of five Champions Tour majors, but Norman is well in contention.
The Australian had three bogeys in his first round, but kept his round on track with six birdies.
But at the top of the charts, 55-year-old Vaughan matched the lowest round of his career and tied the first-round record on Thursday. He won the 2008 Senior British Open for his lone Champions Tour title.
"I've been hitting the ball pretty good lately. The scores just haven't been very good," Vaughan said.
"Today, it just kind of clicked."
Vaughan capped his bogey-free round with a birdie on the par-5 18th, holing an 8-foot putt after hitting his approach into a greenside bunker.
"It wasn't looking very good, to be honest with you," Vaughan said.
"I just executed well and it came off just perfect. When you're playing good, that's the kind of stuff that happens."
Funk had four birdies and a bogey on the final six holes.
"I played pretty solid all day," Funk said. "I had one little hiccup when I missed a little 2-footer on 16, but for the first time, you know how people always say, 'You get it back,' and you always believe you never will, but I made a 50- or 60-footer on the next hole, so I got it back. And then I made a nice putt on 18. It was really solid, and I'm really pleased with that."
Fred Couples, the winner last year at Westchester Country Club in Harrison, New York, was two strokes back at Fox Chapel along with Tom Lehman, Michael Allen and Joe Daley.
Couples fought back problems during the round.
"I feel lucky I got through this round with that score and then I have a lot of rest before tomorrow," Couples said. "I'm really, really stiff though. I don't know exactly what's going on. I'm not getting too much sleep, so I'm just not feeling very well."
Temperatures reached 30 degrees celsius and the heat is expected to remain throughout the weekend to the point that organisers reversed a policy and will permit spectators to bring bottled water onto the course.
Players who teed off early on Thursday said they felt they had an advantage. A relatively short course, the fast greens figure to harden more as they bake as the tournament progresses.
Of the nine players at the top of the leaderboard, six teed off by 9:30 a.m.
Lehman made the most of the preferred conditions coming out of his 8:02 tee time. He birdied the first three holes and was 5 under through seven holes.
"There's a big advantage. Eight o'clock versus 12 o'clock is huge," said Lehman, coming off a victory two weeks ago in the Regions Tradition - another tour major.
Allen seemed to buck that trend slightly, playing the back nine better than anyone at 4-under 31. He was one of only two players to eagle 18. At 567 yards, it is by far the longest on the course.
Allen is the only two-time winner on the Champions Tour this season.
"I was kind of struggling early so it was nice to kind of get it together on the back nine where I played a lot better," Allen said.