Carlos Alcaraz reached the fourth round of the Australian Open for the first time after opponent Shang Juncheng retired with injury in the third set. Remarkably, this was 20-year-old Alcaraz’s first experience of playing a younger player in his 201st tour level match, but it was barely a contest, the second seed dropping just two games before Shang called it a day trailing 6-1, 6-1, 1-0. The 18-year-old Chinese wild card began the match with heavy strapping on his right thigh and took a medical timeout in the second set. Alcaraz was forced to miss the tournament last year with a leg injury and…