American striker Gio Reyna scored a last-gasp winner in Borussia Dortmund's 2-1 victory over Mainz on Wednesday, securing the three points deep into injury-time.

The 20-year-old US international Reyna also scored the fourth in Dortmund's 4-3 home win over Augsburg on Sunday.

On Wednesday he nodded in a header with a minute of injury time remaining, sinking a dogged Mainz.

Dortmund lacked their usual fluency in attack, a consequence of missing the suspended Jude Bellingham.

Bellingham's absence, along with a long-term injury to Marco Reus and Mats Hummels starting from the bench, meant defender Niklas Suele captained Dortmund for the first time.

Manager Edin Terzic promised on Tuesday to fix the side's leaky defence, but Dortmund got off to the worst possible start when Lee Jae-Sung gave Mainz the lead in just the second minute.

Jae-Sung leapt past Julian Ryerson to head in a corner from Edimilson Fernandes, giving the home side their early lead.

Ryerson, playing just his second match for Dortmund after arriving from Union Berlin in January, quickly made up for his error, equalising just two minutes later with a deflected long-range effort.

With two goals after just four minutes, the game looked on course to match Dortmund's 4-3 home thriller over Augsburg from Sunday, but both teams dialled down the intensity for the remainder of the first half.

Terzic made a triple change after 61 minutes, bringing on wingers Reyna and Jamie Bynoe-Gittens -- both of whom scored from the bench against Augsburg -- alongside returning striker Sebastien Haller.

Haller had the crowd on their feet ten minutes after coming on when alone on goal near the penalty spot. But was a step too slow and saw the ball claimed by Mainz goalie Finn Dahmen.

With the match looking to be petering out into a draw, Reyna pounced on Haller's assist, sending the ball past Dahmen and into the back of the net.

Dortmund midfielder Julian Brandt hailed his side's "emotional victory" while Terzic tempered his glee with a reality check.

"With almost the last action (of the game), we are the lucky winner, (but) that could have gone in any direction.

"There is still a lot of work waiting for us."

Mainz's Danish manager Bo Svensson, whose side are only four points clear of the relegation spots, lamented Dortmund's "bitter" late winner, but said "there are many positive things we can take away.

The win sees Dortmund climb to third in the table, with four fixtures still to come this matchday.

In Wednesday's late matches, Union Berlin, Freiburg and Eintracht Frankfurt all have a chance to cut Bayern's lead on top of the table to three points.