The year 2017 saw both Nadal and Federer experience a career resurgency as the two superstars split the four Grand Slams between themselves after returning from injury lay-offs.
The duo also won a combined 13 titles in the calendar year as both occupy the top two in the ATP tennis rankings with Nadal reigning at the top spot.
The Russian questioned the quality of today's youngsters, while John McEnroe recently claimed the next generation was struggling to win titles because of the presence of the Big Four.
However, Safin believes Zverev is quite capable of changing things. The young German has arguably had the most success out of the youngsters in 2017 with five ATP titles and the achievement of a career-high world number three ranking.
The now-fourth ranked star also notably defeated Federer — albeit with the Swiss ace having back problems at the time — in the final of the Rogers Cup in Montreal.
"For me, Zverev, I think," Safin said when asked who could challenge the two long-time rivals.
"Plus he has a good coach, [Juan Carlos] Ferrero. He was a good player and has done good work with him. He is number four in the world, so he can manage, so let's see. The rest [of the young stars], we'll wait for next year."