League leaders Borussia Dortmund maintained their winning form with a 1-0 victory over visiting Cologne in Friday’s lone Bundesliga fixture.Polish striker Robert Lewandowksi ensured the hosts came away with all three points at Signal Iduna Park, capitalising on an extended period of dominance by scoring for the hosts in the shadows of half-time.And the result could have been even worse for Cologne were it not for the efforts of goalkeeper Michael Rensing, who pulled out several acrobatic saves to keep the visitors in the contest in the second half.Dortmund nearly took the lead in the 18th minute through Lucas Barrios, who managed to squirt a shot from a goalmouth scramble only for the ball to be cleared off the line by Kevin Pezzoni.But the hosts kept pressing and eventually found a goal through Lewandowski, who thumped a left-footed volley inside the right post after being played in to the area by Sven Bender in the 41st minute.Milivoje Novakovic could have levelled for Cologne when he raced in alone on a through ball soon after the break, but the Slovenian striker tucked his shot just wide of the right post.The win sees Dortmund extend their unbeaten run to seven games and increase their lead above second-placed Bayer Leverkusen to 15 points, though the latter hold a game in hand.Cologne stay 11th on the table, but could fall as far as 14th when the rest of the weekend’s matches are settled.