Darmstadt picked up their second successive win on the road after they beat Bundesliga strugglers Hoffenheim 2-0 at the Rhein-Neckar Arena on Sunday.
The visitors broke the deadlock through Aytac Sulu just after the half-hour mark and Slobodan Rajkovic secured the victory five minutes from time.
Die Lilien bounced back from a 2-0 defeat at home to Schalke last weekend to move up to 11th in the standings, while achtzehn99 saw their winless run stretch to four outings, leaving them second-from-bottom but tied on 24 points with basement club Hannover.
Dirk Schuster's side made a bright start with Konstantin Rausch testing home goalkeeper Oliver Baumann with a shot from distance on four minutes, while Rajkovic was denied the opener moments later when his header came back off the woodwork.
The hosts' first real chance arrived midway through the first half as Andrej Kramaric went close with a strike from inside the area on the left that flew just wide of the near post.
But they would find themselves trailing shortly afterwards when Baumann flapped at a cross, which fell to Rajkovic and he headed the ball back across the six-yard box for Sulu to score.
Hoffenheim looked to respond but Tobias Strobl and Jeremy Toljan failed to work Christian Mathenia in the visiting goal with their respective efforts from range.
In fact Huub Stevens' men managed just one shot on target in the entire game, which came on 77 minutes as Niklas Sule forced Mathenia into a fine save from inside the area.
They thought Kramaric had equalised with 10 minutes to go after he latched onto a through-ball from Kevin Kuranyi to slot home, but the flag went up for offside.
And Darmstadt would make them pay for their wastefulness five minutes later when Rajkovic netted from close range with the aid of the referee, who got in the way following a corner.