Stefanos Tsitsipas is through to the quarter-finals of the Australian Open for the fourth time in his career after a lengthy battle against Jannik Sinner.The third seed, now the highest ranked man left in the tournament, fended off the Italian for a 6-4, 6-4, 3-6, 4-6, 6-3 victory.Tsitsipas’s quest for a maiden Grand Slam title was hanging in the balance as Sinner made a comeback to take the third and fourth set, but with the re-emergence of his first serve, the Greek dug deep and closed out the match after exactly four hours.‘’It was a long match, guys,” Tsitsipas said during his on-court interview. “I felt like I spent an entire …