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MELBOURNE, Feb 14 (Reuters) - Russian qualifier Aslan Karatsev continued his fairytale run at the Australian Open on Sunday, reaching the quarter-finals on his Grand Slam debut with a 3-6 1-6 6-3 6-3 6-4 win over 20th seed Felix Auger-Aliassime.
Karatsev had not dropped a set in his previous three rounds, including a win over eighth seed Diego Schwartzman, but lost the first two against the Canadian on Sunday at Margaret Court Arena as he struggled with unforced errors.
The 27-year-old took a bathroom break after losing the second set and returned a different player, going for broke and keeping a lid on his errors to win three sets in a row and claim victory in a battle lasting almost three and a half hours.
"It was really difficult in the beginning to play with him. He's a really good player and was playing really fast and it took me two sets to find my rhythm," Karatsev, ranked 114th in the world, said in his on-court interview.
"I put everything into this match and I'm really happy."
Playing the first five-setters of their careers, both players showed nerves but it was Karatsev who displayed better control in the end as 20-year-old Auger-Aliassime began to struggle with his serve.
Karatsev converted his first match point with a forehand crosscourt winner and celebrated with a huge roar as he became the first male player since Alex Radulescu at Wimbledon in 1996 to reach a Grand Slam quarter-final on their debut.
Karatsev also became the lowest-ranked man to reach the Australian Open quarters since No. 114 Patrick McEnroe in 1991 and the first qualifier since Bernard Tomic at 2011 Wimbledon to reach the last eight at a major.
Auger-Aliassime said his serve had let him down and he was unable to compensate.
"Of course I wish I could have served better, but I can't just snap my fingers and it just happens," he said. "So I just got to be a better player overall to overcome these situations.
"The positive thing is mentally I stayed positive ... I believed until the end."
Karatsev will next play 18th seed Grigor Dimitrov, who ousted third seed Dominic Thiem of Austria 6-4 6-4 6-0, for a place in the semi-finals.
"I'll be ready for everyone," said Karatsev, one of three Russians who reached the last 16 this year. Daniil Medvedev and Andrey Rublev will be in action on Monday.