The leaderboard logjam continues after that with 12 players knotted in 13th place at 5-under-par 67, including Ryder Cup captain Ian Woosnam. John Daly, the 2001 champion, leads a group of 13 golfers one stroke further back at minus-4.
The co-leaders combined for 21 birdies at Golfclub Mnchen Nord-Eichenried, compared to just four bogeys.
Cabrera, who won the BMW Championship in May, began his round on the back nine and opened with a bogey at the par-4 10th. He climbed to 2 under with three consecutive birdies, though, before dropping another stroke at the 14th.
The Argentine then made the turn at minus-2 after carding a birdie at the par-3 17th. He made it two birdies in a three-hole span with a birdie at No. 1 before stringing together three straight pars.
Cabrera went on to play his final five holes at 4 under, collecting birdies at Nos. 5 and 9 to go along with an eagle at the par-5 sixth.
'I made a bogey on the first, but apart from that I played very good today,' Cabrera said. 'I am very confident and have played very well. My putter is much better than last year. I think that is the key.'
Derksen, who opened on the front nine, also eagled the sixth hole to get to 4 under. He moved to minus-5 with a birdie at the par-3 eighth before finishing a bogey-free front nine with a par at No. 9.
Two more birdies at the 11th and 13th put Derksen at minus-7, but he dropped a stroke with his only bogey of the round at 15. Derksen then closed with a birdie at No. 18 to beat Cabrera and Rumford into the clubhouse with a 7-under total.
Rumford was the last of the co-leaders to tee off, and like Cabrera, he began his round on the back nine with a bogey at the 10th.
But that would be the last misstep of the round for the Australian, who went on to birdie Nos. 11, 13 and 18 to make the turn at minus-2. Rumford then birdied five holes on the front nine to join Cabrera and Derksen in the lead.
Fourteen players are tied for 38th place at 3-under-par 69. Among them is Irishman and former runner-up Padraig Harrington, who is the highest-ranked player in the field (world No. 11).
Defending champion Miguel Angel Jimenez struggled to a 2-over 74 in his opening round. He is tied with 12 others for 125th place.