Defending US Open champion Novak Djokovic kept his bid for a record 25th Grand Slam title on track on Wednesday after fellow Serb Laslo Djere retired injured while trailing 6-4 6-4 2-0 in their second-round match. Djokovic had needed five sets to get past Djere in the third round a year ago and it looked as though Arthur Ashe Stadium would see another epic clash between the two as Djere went up a break in the second set.