Jansen brace hands Hamburg top spot
Hamburg, on 42 points, moved two clear of second-placed Hoffenheim who drew 3-3 at Stuttgart on Saturday.
Hertha Berlin, also on 40, are third with an inferior goal difference after their 2-1 defeat at Wolfsburg. Leverkusen stay fifth on 36 points, two points behind champions Bayern Munich.
Hamburg were a constant source of danger up front but their defence showed again signs of inconsistency, a point not lost with their coach Martin Jol.
"We have the same problems as Leverkusen and we do not have a perfect team yet," Jol told reporters.
The visitors grabbed the lead against the run of play after 18 minutes when Jansen bagged his first with a powerful left-footer from the edge of the box.
Hamburg captain David Jarolim came agonisingly close to doubling the lead four minutes later but his shot from 16 metres shaved the post.
Leverkusen's in-form striker Patrick Helmes quickly restored order 11 minutes later when he scored, turning and firing a low cross from Manuel Friedrich past keeper Frank Rost for his 16th league goal of the season.
Helmes, who also scored a brace last week against Hoffenheim, could have added a second on the hour when he whipped in a fierce curled free kick but the ball flew just wide.
Leverkusen missed several more chances against an increasingly shaky Hamburg defence, but it was the visitors who struck again in the 67th when unmarked Jansen pounced on a loose Paolo Guerrero header and shot past keeper Rene Adler from five metres.
"Overall I am satisfied with the way the team played," said Leverkusen coach Bruno Labbadia. "Obviously it was clear that after the 2-1 we needed to make some counter attacks."
Hamburg striker Ivica Olic missed a huge chance on a break to make it 3-1 when he only had Adler to beat after a through ball by Guerrero but the Germany keeper stood his ground to deny the Croatian five minutes from time.
Hamburg ended the match with 10 men after Jerome Boateng's second booking for a foul in the 86th minute.