Roger Federer remains the "complete player", Rainer Schuettler has told Omnisport, as the Swiss maestro scales the ATP Rankings once more in record-breaking fashion.
Age is seemingly no barrier to the 36-year-old, who has begun 2018 by sealing a 20th grand slam singles title at the Australian Open and then returning to world number one thanks to his successful Rotterdam Open campaign last week.
Federer is the oldest man to become world number one - Andre Agassi had previously set the benchmark at 33 years.
Schuettler - a finalist in Melbourne in 2003 - was the world number six when Federer first topped the rankings in February 2004 and the German says an ability to adapt his game depending on opponent has been key to his mesmeric longevity.
"He is a complete player," Schuettler said. "You see that, and this is very fascinating for me, against every player he has different tactics.
"He is selecting if he plays with deep slice, more spin, kick or more first serve.
"This is incredible, if he stays more in the back. How he is facing every match tactically, this is the impressive thing for me about his tennis."
Federer's renaissance over the past year has seen him add three grand slam titles to his haul, having previously gone five years without a major triumph as Novak Djokovic and Andy Murray began to dominate at the highest level.
The Swiss and his old foe Rafael Nadal have shared the last five slams between them, as well as topping the rankings, and Schuettler says Federer's comeback and likely continuation of his storied career only adds to his legend.
"When he returned everyone said that it's now the time of Novak and the younger ones," Schuettler added.
"The way he came back is a great accomplishment and I am happy for him as well as for Rafa last year.
"This shows the quality of these two players. I think Roger has so much fun playing tennis so everything is possible.
"As long as he feels fit, as long as he knows he can compete with the best, maybe he won't retire that early.
"I think that everything is possible. I would be pleased if he stays much longer in tennis."
|Oakland Athletics allowing fans to pick price of tickets, all proceeds to charity|
|Ginobili: Spurs not expecting Kawhi Leonard to return this season|
|Trout, Angels brass confident Shohei Ohtani will be a star|
|Osaka wanted to impress Serena with stunning Miami Open win|
|Manchester United apply for professional women´s team|
|Bolt hopes fellow Jamaican breaks his world record|
|Bolt a fan of Ronaldo, but compares his talent to Messi´s|
|Outstanding Osaka knocks out Williams in Miami opener|
|Del Potro ´tired´ heading into Miami Open|
|Heynckes definitely leaving at end of season, claims Ze Roberto|
|Joshua tells Wilder to be ´realistic´ in negotiations|
|Shaw issue a distraction from Manchester United´s failures, says Neville|
|Hamsik withdraws from Slovakia squad|
|RFU backs Jones despite awful Six Nations|
|Hamilton eyes Melbourne pole record, 100 club for Vettel? - Australian GP in numbers|
|Griezmann backs Pogba to prove doubters wrong for France|
|Sanchez ´expected something better´ from Manchester United move|
|WATCH: Thomas conjures up great escape in Austin|
|Javi Martinez in the dark over Spain squad omission|
|Jones should have managed England workload - Flood|
|Giggs feeling the tension ahead of Wales debut|
|Juventus confirm Alex Sandro thigh injury|
|Hart praises ´loyal guy´ Southgate amid World Cup axe fears|
|Windies singing in the rain as they clinch World Cup spot|
|I miss Morata, says recalled Costa|
|I can improve even more at Manchester City - Bernardo Silva|
|Neymar will be fit for the World Cup, insists Ronaldo|
|Costa thinks Griezmann will stay at Atletico Madrid|
|Third DRS zone added for Australian Grand Prix|
|Ferdinand worried by England´s World Cup striker options|
|Hamilton an F1 great - Fittipaldi|
|Stam leaves Reading after wretched run|
|Neymar is great, we´ll see if he joins Real Madrid – Asensio|
|Appalling racism must stop - Inglis|
|Maguire tips ´nightmare´ Vardy to replace injured Kane|
|Red Bull duo: Mercedes still the team to beat|
|Wait and see - Root refuses to confirm new-ball role for Broad|
|Malcom ready to leave Bordeaux and chase Bayern ´dream´|
|Surprised Smith says Rabada appeal verdict draws ´line in the sand´|
|Asensio insists Zidane was not unfair to him|
|Griezmann wants future resolved before World Cup|
|F1 2018: Hamilton v Vettel - the race for five titles|
|Nocerino wants Conte return after Italy´s World Cup ´disaster´|
|Celtics capitalise on Thunder meltdown, Harden scores 42 points|
|Jason Day hungry to reclaim top ranking: My game is close|
|Iconic 2002 World Cup haircut was media distraction, reveals Ronaldo|
|New Zealand star Taylor fit for day-night Test|
|Teenager Anisimova to face Muguruza in Miami|
|McIlroy credits Wayne Rooney for improved putting|
|Stokes return a ´huge boost´ for England – Root|
|Wright: Difficult for Liverpool sensation Salah to resist Real Madrid|
|Yaya Toure yet to report for international duty, confirm Ivory Coast|
|Masters favouritism could hinder Woods, Thomas claims|
|I wish I could still play with him – Ortiz on Red Sox addition Martinez|
|Morrison confirms plans to play for Jamaica|
|Warriors target Friday return for Curry against Hawks|
|Starc and Marsh set to play at Newlands|
|Red alert: Richard Sherman ´excited´ to be a 49er|
|Germany should be proud to have Sane, says Tah|
|Bertrand out of England friendlies with back injury|
|Optimistic McIlroy ´crossed that line´ with Bay Hill win|
|Young backs Mourinho over Shaw treatment|
|BVBolt? Athletics great scores header while training in Dortmund kit|
|Fekir will look to leave Lyon the right way, says agent|
|Ex-Springboks coach Meyer takes charge at Stade|
|Spieth relishing Reed match play showdown|
|Alisson loves Roma and we haven´t discussed Liverpool move, insists agent|
|Henderson becomes third Ireland player to sign new deal|
|Romo ready to create ´mental bubble´ ahead of PGA Tour debut|
|McTominay: I always wanted to play for Scotland|