MELBOURNE, Feb 14 (Reuters) - Australian Open champion Novak Djokovic came through a tough examination of his fitness as the top seed beat Canadian powerhouse Milos Raonic 7-6(4) 4-6 6-1 6-4 on Sunday to reach the quarter-finals.
Djokovic, who injured a side muscle during a five-set win over Taylor Fritz on Friday and suggested he might not be able to continue, showed hints of discomfort on the way to becoming only the second player to reach 300 Grand Slam match wins.
Fourteenth seed Raonic had never previously taken a set off Djokovic in three previous Grand Slam meetings so when he snatched the second set a surprise looked possible.
But Djokovic, bidding for a record-extending ninth title in Melbourne, responded by raising his level and improving his record over Raonic to 12-0.
The Serb will face Germany's Alexander Zverev for a place in the semi-finals.
Djokovic failed to convert break points at 3-3 and 4-4 in the opening set and there was one moment of alarm when he was forced wide on the forehand side and struggled to put on the brakes, hurdling a courtside advertising board.
He won the tiebreak in clinical fashion, though, and when Raonic needed treatment on his right ankle early in the second set Djokovic appeared to be well in command.
Instead, Raonic began to play some of his best tennis and secured the first service break of the match at 2-2 with a powerful forehand winner.
That proved enough for Raonic to bag the second set but his victory hopes were soon receding as Djokovic began to swing freely and accelerated through the third set.
The elastic-limbed Djokovic was stretched in the fourth set as Raonic regained his focus but he broke serve at 4-4 when his opponent netted a backhand and clinched victory a game later to reach the quarter-finals in Melbourne for the 12th time.