South Korean striker Son Heung-Min hit his eighth Bundesliga goal of the season on Friday to seal Bayer Leverkusen's 1-0 win at Borussia Moenchengladbach as the hosts' slump continued.
Sami Hyypia-coached Leverkusen tightened their grip on second place in the table to go seven points clear of Borussia Dortmund, who are at Werder Bremen on Sunday.
Leverkusen are still 10 points behind leaders Bayern Munich, who are at Nuremberg on Saturday, while Gladbach are now winless in their last five games.
Korea's Son hit the winner on 62 minutes with a bullet of a shot off his right foot.
A pass from Germany winger Sidney Sam found Son on the edge of the area to hit the top left-hand corner, giving Borussia goalkeeper Marc-Andre ter Stegen no chance.
"It was a very important game for us as we came in with only a four-point lead, so we're very happy," said Son.
"It was a good pass and I got the ball in a good position, so it gave me space to shoot.
"It's something I often practice in training and I'm delighted it came off.
"We put the work in today and we deserved the three points."
Lucien Favre's Gladbach started 2014 third in the league, but they suffered their three consecutive defeat on Friday which leaves them down in fifth.
Their nightmare run at home to Leverkusen, who they have not beaten in Monechengladbach for 25 years, continued.
The last time they beat Bayer at home was their 2-0 win on February 22 1989, less then eight months before the Berlin Wall fell.
The hosts had their chances with 14 shots on goal, compared to Bayer's 12, and both Germany striker Max Kruse and winger Patrick Herrmann went close.
There was an unusual yellow card handed out by referee Florian Meyer when substitute Philipp Wollscheid was booked while on the bench after only nine minutes.
The replacement defender threw a spare ball into play and was swiftly shown a yellow card.
Leverkusen have a busy programme as they host Schalke 04 next Saturday before Paris Saint Germain visit them in the Champions League's last 16, first round tie on February 18.
