Novak Djokovic has spent most of the last four years as the world’s No. 1 ranked tennis player, but he is not necessarily the people’s champion. Having finally overtaken the affable, urbane superstar Roger Federer and his longtime foil, Rafael Nadal (a popular underdog), Mr. Djokovic knows the crowd is not always on his side. But that has only steeled his resolve.

During the Rogers Cup tournament in Montreal last month, he stepped into an empty space adjacent to the media room at the Uniprix Stadium complex in Jarry Park. He was flanked by his agent Elena Cappellaro and a publicist for the ATP tour. For a 6-foot-2 elite athlete, Mr. Djokovic seemed startlingly slight.

In his 2013 book, “Serve to Win,” he wrote of dropping from 182 pounds to 172 in the wake of switching to a gluten-free diet, causing his family and friends to worry he was becoming too skinny. But far from seeming fragile when you see him up close, Mr. Djokovic projects strength and sturdiness. When he sat down, his posture was perfect.

“At the beginning of my career,” said Mr. Djokovic, 28, “I was observed as a rebel, somebody who comes in the mix and starts to challenge the two guys that were so dominant. ‘Who is this guy from Serbia, small country, comes from nowhere and starts saying he can beat the top guys and become No. 1?’ Obviously, I understand the people’s reaction to that, but I felt that the only way for me to get out there was to show that I deserved to be there.”