Winner of the Jede Hot Cup and Stiga Open on the Challenge Tour in 1991, with both victories coming in his native Sweden, Hallberg is playing in his first Staysure Tour Qualifying School since 2015.
The 53 year old, who started his round from the tenth tee, birdied the 16th and 18th holes to make the turn two under par, and then made consecutive gains on the first and second holes to move to four under.
A bogey on the par three third hole was cancelled out by his fifth birdie of the day, on the par four seventh hole, to come back in 32 shots for a one stroke lead ahead of England’s Mark Booth. Hallberg is one of six players in the 75-man field to score better than par.
“I was lucky at the times when I needed to be lucky,” said Hallberg. “I took advantage on the easier holes when they were downwind.
“I don’t have any expectations going into this week. I’m surprised I’m top of the board at the moment.
“I’ll be drawing from my experiences on the Tour when I was in contention, but it’s been a long time since I stopped playing. I need to keep playing my game, with my tactics, and not rush myself if I get into trouble.”
Booth, who made it to Final Stage in 2017, started his round with a birdie on the first hole and then birdied the ninth to make the turn two under par. He then birdied the 12th and 14th holes, but found a fairway bunker on the 18th hole, and was forced to settle for a bogey.
“Coming through First Stage was tough, it was really difficult out there last week,” said Booth. “Today it was similar, the greens were tricky and you had to pick your spots. It’s that sort of golf course, if you’re out of position you will get punished.
“I was really unfortunate on the last hole, I hit a good tee shot but didn’t quite fly the trap and went into the bunker. Apart from that I was flawless.
“I’m playing well enough, there’s no reason I can’t be up there at the end of the week. I’m looking forward to it.”
England’s David Shacklady, who missed out on one of the five coveted cards after losing in a play-off at last year’s Final Stage, is two strokes behind Hallberg on two under par.
Spain’s José Buendia, Argentina’s Mauricio Molina and Englishman Paul Streeter share fourth place on one under par.
The top five golfers at Qualifying School will earn playing rights on the Staysure Tour for the 2018 season. After Wednesday’s third round there will be a cut, with all players within eight shots of fifth place making it through to Thursday’s final round.