Tennis: Djokovic cruises to semi-finals

Novak Djokovic has cruised into the Australian Open semi-finals after his opponent Kei Nishikori retired mid-match with a leg injury. And Serena Williams has crashed out of the tournament, rolling her ankle and squandering four match points and a 5-1 lead against Czech opponent Karolina Pliskova. Our correspondent in Melbourne is David Luddy.