MELBOURNE, Jan 28 : Novak Djokovic said on Wednesday that the off-season can be a “two-edged sword”, with the physical and mental workload players undertake to improve their game sometimes leaving their bodies under strain before the new season even begins. The 38-year-old marched into the Australian Open semi-finals in search of a record-extending 11th title at Melbourne Park and a standalone 25th Grand Slam crown, but his progress has come amid a streak of unfortunate injuries for his opponents. Italian fifth seed Lorenzo Musetti was forced to retire from their quarter-final on Wednesday despite leading 6-4 6-3 1-3, a round after Czech 16th seed Jakub Mensik also withdrew with injury. “There’s been a lot of talk about the schedule,” Djokovic told reporters. “From one point of view, you would say, well, the players have rested and trained to get themselves into a good physical, emotional, mental game state to perform well and have no reason for an injury. “But it’s kind of a two-edged sword. The off-season is the only time you can really …