FORMER world No 1 Roger Federer yesterday cast off doubts about his fitness by triumphing in a five-set thriller with Kei Nishikori to reach the quarter-finals of the Australian Open.

It was only Federer’s fourth competitive match since returning from six months out with injury but the Swiss proved resilient on Rod Laver Arena as he progressed with a 6-7 (4/7), 6-4, 6-1, 4-6, 6-3 win lasting three hours and 24 minutes.

He will continue his pursuit of an 18th grand slam title against Mischa Zverev, the world No 50 who earlier shocked top seed Andy Murray to reach the fifth round.

Federer – who watched Zverev secure his dramatic win – last played in a five-set match at Wimbledon last year, losing to Canada’s Milos Raonic at the semi-final stage. That came just two days after the Swiss won a decider against Marin Cilic.

The 35-year-old will be hoping to avoid a similar hangover this time around, particularly given how the draw has opened up for him following Murray’s early exit.

It had seemed like Federer would succumb similarly as Nishikori stormed 4-0 up in the first set and held on in the tie-break, only for the Japanese to be forced back on to the back foot. His form dipped after he received treatment on a back injury in the fifth set, in particular.

“He was playing his heart out,” Federer said. “I thought it was a great match and joy to be a part of it.

“It was about staying calm, after almost going down 6-0, I thought it wasn’t getting any worse from there. It paid off in the end somehow.”