Roger Federer, now 37, remains the chief threat to Novak Djokovic. © AFP Roger Federer and Novak Djokovic are both gunning for a record seventh Australian Open crown from Monday, but Andy Murray will make his last appearance in Melbourne as the era of the “Big Four” draws to a close. Number one Djokovic and third seed Federer face a stern challenge from the likes of youthful force Alexander Zverev, seeded four, who is still looking for a first major to cement his place as torch-bearer for the next generation. But Murray dropped a pre-Grand Slam bombshell, breaking down during a tear-filled press conference as he revealed chronic hip pain means he will retire after Wimbledon – if he can carry on that long.And question marks remain over the fitness of world number two Rafael Nadal who pulled out of his Brisbane warm-up but arrived in Melbourne professing he was “fully fit” and promising to unleash a remodelled serve.It all means the era of the “Big Four” is almost over after a season in which Federer – who opens his title defence against Denis Istomin on Monday – rolled back the years on Rod Laver Arena to lift an emotional 20th Grand…