Craig Parry, Rory Sabbatini, Joe Ogilvie and Tom Pernice, Jr., are all one back after shooting 8-under 64s.
Couples matched his lowest score of the season. The 42-year-old shot 7-under 63 in the second round of this years Byron Nelson Classic, where he eventually finished tied for 11th ' his best showing of the season.
Meanwhile, Lehman shot his lowest score since he opened in 63 at the 2000 Phoenix Open; which, coincidentally, was his last victory.
Couples last PGA Tour title came at the 1998 Memorial Tournament. The victory was part of a two-win season and a 9th place finish on the money list. The other win came at the Bob Hope Chrysler Classic, which, along with Vegas, are the only 90-hole events on the current PGA Tour schedule.
Since his Memorial triumph, the 14-time Tour winner has failed to record a top-3 finish. Two years ago, Couples also shot a first-round 63 in Vegas, but faltered with a Saturday 79 and tied for 10th.
He said earlier in the year that he didn't know if he still had the desire to be competitive on the tour. He even hinted at retirement.
'I don't want to quit,' Couples said. 'I hate to have people think this guy is having one bad year and he's ready to quit. That's not the case.'
Wednesday, Couples carded nine birdies and no bogeys at Southern Highlands. He creditted his score to a new putter he anchors against his stomach.
'I don't want to say I had the yips, but my stroke wasn't as solid as it should be,' said Couples, who took 28 putts in the first round. 'I needed to be better on the greens, that's the bottom line.'
Five of the top six players on the leaderboard played Southern Highlands in the first round; Lehman was the lone exception, playing the host course, the TPC at Summerlin.
The third course is the TPC at The Canyons. The 144-man field will play each of the three courses over the first three rounds alongside amateurs. A cut will then be made (top-70 and ties) and only the remaining professionals will compete on Summerlin over the weekend.
'These aren't the hardest courses,' Couples said. 'Obviously, I'm surprised at 63, but very excited how I putted the ball.'
Its been 58 PGA Tour starts since Couples entered the winners circle, miniscule when compared to that of Parry. The 35-year-old Australian has yet to win on tour, a span of 217 tournaments.
Parry has earned $4,314,312 since playing his first PGA Tour event in 1987. He held the dubious distinction of having earned the most money without a Tour victory, before being past this season by Flesch.
Flesch, 34, is in his fifth full season on the PGA Tour and has collected $4,338,149.
Full-field scores from the Invensys Classic