Goosen, the 33-year-old ranked fourth in the world, started his round on the back nine and quickly preceded to birdie four of his first six holes.
Currently second on the European Order of Merit, Goosen tallied seven birdies for the day, but also carded two bogeys on the eighth and 15th holes. However, he erased the two bogeys when he chipped in for eagle on the par-5 16th to close out his scoring for the day.
'Obviously, I'm keen to regain the title,' said Goosen, referring to the Order of Merit. 'It all depends what Ernie does, but there's a chance for me.'
Leaney, who had a strong sixth-place showing last week, got off to a solid start but really heated up at the turn. Birdies on 10, 11, 12, and 13 got him to 6-under, and another on the 15th forged a tie at the top with Goosen.
'It was nice to hole some putts,' said Leaney, a three-time winner on the European Tour. 'I got off to a good start and kept it going. I hit a lot of good iron shots and most of my putts were 12-18 feet. Had two-putt birdies on the 13th and 15th. Just a good solid day.'
His last win at the 2000 TNT Open had started to to become a distant memory. But Leaney knocked home a birdie on the closing hole to gain the outright, and overnite, lead at 8-under. He'll be hoping to duplicate Robert Karlsson's wire-to-wire win at last week's European Tour stop.
Mark Pilkington of Wales had eight birdies with a lone bogey and carded a 7-under 65 and a share of second place with Goosen.
Gary Evans, who made headlines with his wild finish at this year's British Open, is one of 10 players three behind the leader at 5-under. His finish today was anything but wild as he parred the last seven holes, and eight of the nine coming in.
Also at 5-under is European Ryder Cupper Thomas Bjorn, who nearly aced the par-3 11th as his tee shot rolled six inches past the cup.
American Rich Beem, the newly-crowned PGA champion, played his first round on continental Europe and shot a 1-under 71. He birdied four of his first seven holes, but a couple of bogeys and a double bogey left him seven off the pace.
Last week's winner in Switzerland, Robert Karlsson, continued his good play in getting to 5-under-par through 17, but an unfortunate bogey on his last hole dropped him back to 4-under and a share for 14th.
Four-time winner of the event - and co-promoter - Bernhard Langer enjoyed another good start and kept the leader in his sights with a 4-under 68.
Ernie Els, playing alongside host Langer, posted a 3-under 69 despite a bogey on the opening hole.
Full-field scores from Linde German Masters