Novak Djokovic moved a step closer to retaining the Australian Open title with a straight-set semi-final win over injury-hit rival Roger Federer.Serbia's Djokovic, 32, was 4-1 and 40-0 down in the opening set before recovering to win the tie-break.That laid the platform for the second seed to go on and win 7-6 (7-1) 6-4 6-3.Djokovic, aiming for a 17th Grand Slam title, will face Dominic Thiem or Alexander Zverev in Sunday's final.Victory would mean a record-extending eighth Australian Open triumph for Djokovic, who has lost just three matches at Melbourne Park in the past 10 tournaments.Importantly, it would also move him within three more of 38-year-old's Federer's all-time record of 20 Grand Slam titles and two of Spain's Rafael Nadal, who lost to Thiem in Wednesday's quarter-final.