Roger Federer expects to see Novak Djokovic challenging for grand slams and top spot in the rankings after slipping out of the world's top three for the first time since 2009.
Djokovic saw his defence of the French Open come to an end at the hands of Dominic Thiem at Roland Garros last week, leaving him with just one tournament victory this year, at the Qatar Open in January.
The Serbian's extended dominance of the men's game has crumbled in the last 11 months, an early exit from Wimbledon and defeat to Stan Wawrinka in the final of the US Open allowing Andy Murray to replace him as world number one.
Speaking before his return to action at the Stuttgart Open, Australian Open winner Federer acknowledged the extent of the unexpected difficulties being suffered by Djokovic, who parted company with his coaching staff last month and began working with Andre Agassi in Paris.
"When compared to a year ago, when he [Djokovic] held all four Grand Slams, he's in a relatively difficult situation," Federer said at a news conference.
"But, at the same time, he is still in the world's top four, in the race for titles, and easily inside the top 10.
"But there is no doubt that he has been weak for a player with his abilities. I guess it is not so bad, but it really depends on how you view his situation.
"Clearly, last year was very good for him, especially during the first half of 2016. So it's quite surprising that he hasn't pushed on since winning the French Open last year.
"I have to admit that I've seen him play very little recently and I cannot judge his situation. Most of what I have noticed, has come through the media.
"But he is too good a player for this to continue indefinitely.
"He will win grand slams in the future and fight again for world number one. So I expect to see a strong Djokovic in the future."