Borussia Dortmund's struggles in the league continued with a 4-2 loss at Holstein Kiel earlier on Tuesday, raising the heat on under-fire coach Nuri Sahin.

In Leverkusen, Xabi Alonso's defending champions were dominant against an in-form Mainz but were held at bay until Grimaldo produced a stunning free kick after 48 minutes.

The Spain defender lifted the shot over the Mainz wall and beyond the outstretched palm of Robin Zentner, his 12th goal on his 50th Bundesliga appearance.

Florian Wirtz, who won the free kick, along with Nathan Tella and Jeremie Frimpong all had late chances saved.

Leverkusen's 10th straight win in all competitions handed down the gauntlet to Bayern, who face Hoffenheim at home on Wednesday.

Mainz, who lost coach Bo Henriksen to a second yellow for backchat in stoppage time, stay in fifth despite the loss.

Eintracht Frankfurt put on a second-half display to beat Freiburg 4-1 at home, with Omar Marmoush again getting on the scoresheet.

Marmoush, heavily linked with a winter move to Manchester City, scored Frankfurt's second, while Robin Koch, Hugo Ekitike and Nnamdi Collins all got on the scoresheet for the home side.

Wolfsburg thumped Borussia Moenchengladbach 5-1 at home, with Lukas Nmecha scoring two goals in four second-half minutes.

The Wolves, cruising under former Southampton and RB Leipzig boss Ralph Hasenhuettl, have now won six of their last eight to sit in sixth.

- Sahin feeling the heat -

Dortmund were heavily favoured against their promoted opponents, who sit second-last in the table, but were overrun as Kiel scored three goals in 22 first-half minutes.

Shuto Machino, Phil Harres and Alexander Bernhardsson found the net to have Kiel up by three at half-time.

Dortmund's Gio Reyna and Jamie Gittens scored in the second half but the visitors could not pull off an unlikely comeback, with Jann-Fiete Arp scoring Kiel's fourth in stoppage time.

Captain Emre Can said Dortmund's woes were "nothing to do with the coach -- we as players are responsible".

"With all due respect to Kiel, we can't play like that. It's a matter of honour," the 31-year-old added.

Kiel have now won two of their last three to boost their chances of avoiding a direct relegation.

With half the season played, Dortmund sit ninth, 14 points behind league leaders Bayern Munich.

Questions will continue to be asked of coach Sahin, who replaced Edin Terzic in the summer, despite the latter taking Dortmund to the Champions League final in June.

With want-away forward Donyell Malen joining Aston Villa just an hour before kick-off, Sahin handed teenage forward Julien Duranville a starting XI debut.

In cold, foggy conditions on Germany's northern coast, Dortmund dominated possession for much of the first half-hour, but were unable to break through the dogged hosts.

With 27 minutes gone, Kiel forced Julian Brandt into an error near his own penalty box, Bernhardsson then found Machino who blasted in the opener.

Harres, a fourth-division player this time last season, doubled Kiel's lead with a clever header on the counter shortly afterwards.

Kiel hit a third just before half-time, Bernhardsson tapping in a Harres cross to have Dortmund reeling.

Sahin made four attacking changes in the opening 15 minutes of the second half as Dortmund pursued an unlikely comeback.

But despite goals by Reyna and Gittens, the away side were unable to find a third, even as Kiel's Lewis Holtby was red carded with four minutes remaining for a rough challenge.

Local boy Arp scored in the eighth minute of stoppage time to seal a famous Kiel win.