Marat Safin has questioned the strength of the next generation in men's tennis after Roger Federer and Rafael Nadal dominated the 2017 ATP Tour.
After struggling with injuries last year, Federer and Nadal have enjoyed a resurgence over the past 12 months – sharing the four grand slams between them.
Federer finished 2017 with 52 wins from 57 matches, while Nadal ends the year as world number one for the first time since 2013.
Nadal – who won the French and US Opens – racked up an impressive 67 victories, before knee problems hampered his progress at the ATP Finals.
While the tennis world - including Safin - marvelled at their return, the two-time grand slam champion questioned why the Tour's younger players were not providing more of a challenge.
Speaking to Omnisport ahead of the Champions Tennis at the Royal Albert Hall, Safin said: "How is it possible that they [Federer and Nadal] are still winning? Still crushing everybody and chopping everybody off?
"They are superstars, but what is wrong with the juniors? They need to start to bite!"
One player that has impressed Safin is world number four Alexander Zverev, though, and he believes the 20-year-old can compete in 2018.
Safin added: "For me Zverev [has done well], Zverev plus his good coach [Juan Carlos] Ferrero, he did good work with him, he got to three in the world so he can manage. Let's see.
"The rest of them, we will wait for next year."