MONTREAL - Top seed Novak Djokovic claimed his 50th win of the season with a 6-2, 6-1 victory over Jack Sock to reach the Montreal Masters quarter-finals Thursday.

It was the world number one's 28th consecutive win at the Masters level. His victory, achieved in just 54 minutes, gave the Serb a last-eight match-up against either Ernests Gulbis of Latvia or Donald Young of the United States. The win was straightforward for three-time Canadian champion Djokovic, who broke Sock of the United States four times to win their first meeting. Djokovic has won six titles in 2015 -- the Australian Open and Wimbledon as well as four Masters events.

MURRAY BEATS ROBREDO AFTER DELAY, WAWRINKA EXITS MONTREAL: Andy Murray took care of some unfinished business as the second seed advanced to the third round of the ATP Tour's Montreal Masters on Wednesday with a 6-4, 7-5 win over Tommy Robredo.

The Scot ended the match, which was postponed because of heavy rain Tuesday, by executing a drop shot which bounced on the top of the net before falling onto Robredo's side of the court for the winner. But there was disappointment for French Open winner Stan Wawrinka who was unable to carry on due to apparent back problems, with the Swiss quitting to hand a 6-7 (8/10), 6-4, 4-0 victory to Australian Nick Kyrgios.

Wawrinka, who did not play after Wimbledon due to a shoulder injury, called for the trainer as he trailed two breaks in the third set. After speaking with the trainer and his coach Magnus Norman, the 30-year-old third seed decided to give up the match. Murray set up a meeting with Gilles Muller -- whom he has defeated twice this season -- as he finished his second round contest with 24 winners and 30 unforced errors and breaking four times. The pair resumed play Wednesday after standing 4-4 in the opening set when the rain stopped play on Tuesday night.

Rafael Nadal made an encouraging start to the summer hard-court campaign as the Spaniard recovered to defeat Sergiy Stakhovsky 7-6 (7/4), 6-3.

Japanese fourth seed Kei Nishikori, the winner of the Washington title last weekend and now ranked fourth on the ATP Tour, started 6-3, 6-3 over Spain's Pablo Andujar in 70 minutes. Nishikori made a successful comeback last week after a long-running thigh injury and said that his fitness is good.

Donald Young surprised fifth seed Tomas Berdych 7-6 (7/5), 6-3 while tenth-seeded holder Jo-Wilfried Tsonga had to fight to survive 5-7, 7-6 (8/6), 7-5. American Jack Sock sprang an upset on troubled 14th seed Grigor Dimitrov, who went down 5-7, 7-6 (7/5), 7-5. Over recent weeks, the Bulgarian once tipped as a future Roger Federer, lost both his Australian coach Roger Rasheed and Russian girlfriend Maria Sharapova.