Carlos Alcaraz sent a chilling warning to his Australian Open rivals on Saturday, outclassing Shang Juncheng before the Chinese teenager was forced to retire as Iga Swiatek aims for her 19th straight win. A day after defending champion Novak Djokovic returned to top form, world number two Alcaraz laid down a marker in a lopsided third-round contest lasting just 66 minutes. The 20-year-old, playing a tour-level match against a player younger than himself for the first time in his career, was leading 6-1, 6-1, 1-0 when his 18-year-old opponent had to pull out.