A crucial early Sunday fixture at the Coface Arena had relegation implications for visitors Hamburg, while the hosts looked to mount a late European challenge.
Freiburg' defeat to Paderborn further tightened up the bottom five, and HSV knew a win would lift them to the lofty heights of 14th.
Early on the game was played at a high level of aggression with both van der Vaart and Kaçar, the former due to the referee, sustaining injuries before the five minute mark. Both were able to continue.
It was almost 1-0 in the 9th minute, but the aforementioned Kaçar was able to scoop the ball away from the six yard box after good ball movement by the Reds.
Baumgartlinger has a stinging drive in the 19th minute well saved by Rene Adler as the home side piled on the pressure. At the other end there was a lack of composure and quality from Hamburg despite the positions they had gotten in to.
HSV were happy to sit back and absorb pressure, because while the hosts' record of 5 wins, 6 draws and 4 defeats was nothing to shout about, Hamburg had just two wins away from home all season.
Ivica Olic was a source of threat for the Dinosaurs, having two chances in the 23rd minute to cross. The first was behind Lasogga who had made a good run, the second almost met the former Hertha target man but was well saved by Karius.
Elkin Soto was injured in a challenge with van der Vaart right on the half hour mark. It didn't look good live, but on replay his right leg went under his body very awkwardly, and he left the field to a standing ovation on a stretcher. Yunus Malli came on in his place, and the Dutchman was given a real berating from the home crowd who booed his every touch.
The visitors took a shock lead in the 37th minute in bizarre circumstances. Westermann cut in from the RB position and had his cross deflected by Baumgartlinger, and the ball looped over Karius in to the top left corner. The travelling support was sent into raptures as Labaddia's side had a crucial goal.
Nikolai Müller went off just minutes after the goal with an injury, replaced by Jansen, and in the process not even making it to half time against his former club. Jansen, along with van der Vaart, was making 150th appearance in the league for HSV.
Hamburg had a big chance to double their lead in the first minute of stoppage time as Lasogga squared across the edge of the box to Jansen on the break, but his effort was straight at Karius as the chance went begging.
The first half ended with the score 0-1. An element of fortune the main contributor to their first 45 success, but they will definitely take it in their attempts to preserve eternal Bundesliga status.
Hamburg started the second half well and had a penalty shout inside the first minute, but the foul on Jansen was adjudged to be outside the area by about a yard.
In the 55th minute Olic had a chance over on the left side on a break for Hamburg, but his effort from a narrow angle was blocked. The resulting corner was met by Kaçar but was well saved by Karius in the Mainz goal.
The game descended into desperation rather than quality, with both teams eager for the three points for different reasons. Lasogga hit a shot in the 67th minute that drew a good low save, but aside from that the chances were non-existent heading into the last 20 minutes.
However, in the 69th minute, the hosts must have thought they were level. Koo Ja-Cheol nearly equalised with a low shot, but it hit the base of the near left post, as Mainz were unable to make Hamburg pay for the 3-on-2 situation they had allowed to develop.
Mainz did level with 15 minutes left as Yunus Malli slotted home after a great feed from the byline, easily beating a wrong footed Adler with a cool finish. Suddenly, what may have been three points turned into a dangerous situation for Hamburg, who knew only a win would take them out of the bottom three.
Rene Adler was called into action again with eight minutes left as he outstretched his paw to tip a wonderful free kick away.
It was Hamburg who looked to have found the winning goal in dramatic fashion with just three minutes left. Mainz failed to clear a corner and the ball fell to Kaçar, who at the second attempt swept the ball past Karius, making it 1-2 and sending the visiting supporters wild again.
The drama continued as just a minute later Rudnevs was set through on goal by Lasogga, courtesy of a clever back heel. Rudnevs was dragged down by the back tracking defender, and it was a straight red card for Brosinski, making the situation for Mainz even more difficult.
Three minutes were added at the end of the match, and they passed without incident, meaning HSV had secured arguably the most vital three points of their season.
The visitors moved to 14th with the win, overtaking Paderborn and pushing Freiburg and Hannover to 16th and 17th respectively. Mainz stayed on 37 points, effectively ending any shot at the top 6, but the story of the day was Bruno Labbadia's sides heroics that may just save them for another year.