Murray's reign as World No. 1 is under threat, believes Federer

18-time Grand Slam champion Roger Federer feels that Andy Murray's reign as the world number one is under serious threat for the rest of the summer.

Murray's troubled season continued as he slumped to a shocking straight-sets defeat at the hands of Australia's Jordan Thompson in the opening round of the Aegon Championships on Tuesday.

The World No. 1 was hoping for a good preparation for his Wimbledon title defence by winning the Queen's Club title for a sixth time.

However, he went on to suffer the most embarrassing defeat of his career, losing 7-6 (7-4), 6-2 to world number 90 Thompson in a clash that lasted one hour and 43 minutes.

Murray's defeat ended his 14-match winning streak on grass and his 10-match unbeaten run at Queen's Club, a warm-up event to Wimbledon.

"Andy was tired after getting to No1 at the end of last year and then has been struggling with injuries," The Guardian quoted Federer as saying in Halle on Thursday before his fighting 7-6 (7-4), 6-4 win over Mischa Zverev in the ongoing Gerry Weber Open.

The Swiss Maestro needed 90 minutes on the grass courts of the German city to defeat Zverev, who had dumped Murray out of the Australian Open in the fourth round earlier this year.

"I think the second part of the season will be really crucial for Andy, and Novak [Djokovic] as well. I think the second half of the season is going to be really, really interesting," added Federer, who is returning after taking 11 weeks off the Tour to prepare for his pursuit of an eighth title at Wimbledon.