Spain's Rafael Nadal has entered the Men's Singles quarter-finals of the Australian Open tennis tournament. In Melbourne today, Nadal overcame a tenacious Argentine, Diego Schwartzman, in four sets 6-3, 6-7, 6-3, 6-3 in a match that lasted almost four hours. By winning, Nadal ensured that he will remain world number No.1 when the new rankings are released the day after the Open ends. Nadal will next face Croatia's sixth seeded Marin Cilic of Croatia.