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."
Champions Tennis at the Royal Albert Hall, is running from Thursday 30th November – Sunday 3rdDecember 2017. http://championstennis.co.uk/
|Atletico Madrid 2 Athletic Bilbao 0: Gameiro and Costa settle cagey clash|
|NHL commissioner Bettman condemns ´reprehensible´ Smith-Pelly abuse|
|Visser, Scott and Nel among six recalled by Scotland|
|Bhuvneshwar and Dhawan inspire India´s Wanderers win|
|Napoli set club record with another Serie A win|
|Froome disappoints in time trial as Wellens wins Ruta del Sol|
|Napoli 1 SPAL 0: Allan sends hosts top of Serie A again|
|Federer makes short work of disappointing Dimitrov|
|Allegri allays Higuain injury fears|
|Blackhawks fans ejected after racist heckling of Capitals´ Smith-Pelly|
|Allegri convinced Juve can get past Spurs|
|Sri Lanka sweep series after dominating Bangladesh|
|Everton fear ´long-term´ injury for Mangala|
|If it isn´t Madrid, it´s Paris - Hazard tires of speculation|
|Luiten claims Oman Open title|
|Torino 0 Juventus 1: Champions win derby but lose Higuain|
|Neymar exit made Barcelona stronger - Ter Stegen|
|Remove T20 cricket from international schedule - Bayliss|
|Injured Higuain forced off against Torino|
|Jesus confident of returning strong after knee injury|
|DeGale launches Twitter attack on beaten Eubank Jr|
|We had to change hotels – Ovrebo recalls fury after 2009 Chelsea-Barcelona semi-final|
|Spalletti: Inter board will decide my future|
|Munro blitz helps Black Caps progress despite defeat|
|Fabregas still wary of timeless Iniesta|
|Liverpool´s 30-goal hero Salah: There´s more to come|
|Valverde: Coutinho dropped for the good of Barcelona|
|Garcia floors Rios for ninth-round TKO|
|Ronaldo better than Neymar? No says Xavi|
|Donovan Mitchell caps NBA All-Star challenges with dunk success|
|Anderson outlasts Nishikori to reach New York Open final|
|Steve Nash, Ray Allen among Basketball Hall of Fame finalists|
|Kevin Love: It was necessary to clean house in Cleveland|
|Real Madrid hurt you with nothing – Xavi says PSG loss an injustice|
|Conte: Chelsea need perfect game to stop Messi and Barcelona|
|LeBron James thanks Ingraham as Cavs All-Star vows to speak his mind|
|Archer says Rays could have best rotation in baseball|
|Bubba Watson takes lead with 65 at Genesis Open|
|Inter in a fragile moment, says Spalletti|
|Groves batters Eubank Jr to retain WBA crown|
|Montpellier see off plucky Oyonnax, Ashton stars in Stade mauling|
|Booed Emery praises PSG´s mental strength|
|Cavani hails ´good reaction´ as PSG bounce back in style|
|Thiem breezes past Monfils to set up Bedene showdown|
|Mourinho wants referees to have final say on VAR|
|Genoa 2 Inter 0: Pandev piles misery on old employers|
|Manchester United to host Brighton in FA Cup quarter-finals|
|Leinster see off Scarlets as Munster come up short in Cardiff|
|New No.1 Federer earns gritty Seppi win to book Rotterdam final|
|Mata ´still up for VAR´ despite disallowed goal at Huddersfield|
|Valverde: Barcelona had to suffer at Eibar|
|Championship Review: Fulham end Villa run, Cardiff pounce|
|Resilient Kvitova ousts Wozniacki in Doha thriller|
|Huddersfield Town 0 Manchester United 2: Lukaku sends visitors through after VAR controversy|
|Toomua inspires Tigers win over Quins, Worcester beat Gloucester|
|Bayern Munich boss Heynckes hails game-changing Muller|
|Locadia benefiting from ´competitive´ Brighton - Hughton|
|Baby on board? Alba dedicates Barca goal to girlfriend|
|West Brom future ´not a decision for me´, says Pardew|
|Lions get De Bruin off to winning start|
|Paris Saint-Germain 5 Strasbourg 2: Cavani at the double in bounce-back win|
|Bar-celona! Messi posts unwanted record|
|Barca go 31 LaLiga games unbeaten to tie club record|
|Mourinho: I don´t know and I don´t care when Pogba returns|
|Brighton and Hove Albion 3 Coventry City 1: Locadia, Ulloa lead Seagulls to quarter-finals|
|EIbar 0 Barcelona 2: Suarez and Alba seal points as Orellana sees red|
|West Brom 1 Southampton 2: Rondon stunner in vain as Baggies drop out|
|Wolfsburg 1 Bayern Munich 2: Lewandowski spot-on after Robben´s penalty miss|
|I hope Cutrone finds a beautiful girlfriend so he can rest and make love - Gattuso|
|Wellens claims Ruta del Sol lead, Froome suffers puncture|