Andy Murray beats Mikhail Kukushkin at Australian Open 2012

Andy Murray reached the Australian Open quarter-finals for a third year running after Mikhail Kukushkin retired through injury while trailing 6-1 6-1 1-0.

Two five-set matches in rounds two and three took their toll on Kazakhstan's world number 92 Kukushkin, who pulled out with a left hip flexor problem.

"It was hot on the court and it is good for me that I have conserved energy," said British number one Murray.

The fourth seed will play Kei Nishikori after he stunned Jo-Wilfried Tsonga.

Murray has a winning record against the 24th seed, the first Japanese man to reach the Australian quarter-finals for 80 years, and the Scot approaches that match on Wednesday feeling fresh.

He described the 49-minute win over Kukushkin, 24, as "boring" and stated his intention to return to Rod Laver Arena later on Monday for an extra practice session.

"I played him [Kukushkin] a few weeks ago at Brisbane and it was three tough sets and I expected another tough one today," said the Scot.

"I hadn't seen him play before Brisbane and he hit some huge shots early and made me work.

"It is tough for him for that to happen in the fourth round of a Grand Slam for the first time."

As in his third-round victory over Michael Llodra, Murray, 24, played closer to the baseline and attacked his groundstrokes with plenty of aggression.

Kukushkin beat Gael Monfils in five gruelling sets on Saturday and those exertions seemed to hinder him in the scorching Melbourne heat.

Three unforced errors in the first game handed Murray a break and the Scot struck again for a 3-0 lead.

Kukushkin finally displayed some fight to hit back in game four, only to relinquish his serve again after pushing two backhands wide.

The world number 92 gave an indication he was not 100% fit when he called for the trainer at the next change of ends, and Murray broke once more to wrap up the 22-minute first set.

A lapse in focus saw the world number four relinquish an early break in the second set with a poor backhand into the net.

But a struggling Kukushkin, who started rushing the net in a bid to shorten points, handed the momentum straight back.

Murray breached his defences with some clean hitting to go 5-1 up before serving out another 22-minute set to love.

Having required more treatment and then lost his serve for an eighth successive time, Kukushkin called it quits.

Next up for Murray is Nishikori, who stunned sixth seed and 2008 runner-up Tsonga 2-6 6-2 6-1 3-6 6-3 in three hours and 30 minutes.

"I'm feeling unbelievable," Nishikori said. "To reach my first quarter-final and beating Tsonga makes me really happy.

"Hopefully it's big in Japan. A lot of people messaged me when I got to last 16. Now I'm in the quarter-finals, it's really exciting."

In the mixed doubles, Britain's Colin Fleming reached the quarter-finals with his partner, the South African-born American Liezel Huber.

The duo defeated American Steve Lipsky and Vladimira Uhlirova of the Czech Republic 6-3 6-2.