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."
|I want to fight the champion! - Joshua reiterates Wilder in April ambition|
|Gundogan glad to leave Christmas holidays behind|
|I know how it feels and it hurts - Gundogan on Sterling abuse|
|Celtic 1 Red Bull Salzburg 2: Scottish champions through despite defeat|
|Olympiacos 3 AC Milan 1: Rossoneri crash out of Europa League|
|Arsenal 1 Qarabag 0: Lacazette on target as Saka shines|
|Hansen to leave: All Blacks will take time to name replacement|
|Chelsea condemn alleged anti-Semitic chanting from fans|
|Hansen to leave: The best Opta facts from New Zealand coach´s tenure|
|Hansen to leave: Five contenders to be the next All Blacks coach|
|Hansen to leave: Highs and highs of his All Blacks reign|
|Hansen to leave All Blacks after World Cup|
|Rapid Vienna 1 Rangers 0: Late Ljubicic winner sends Gerrard´s men out|
|Beckham continues to miss Giants practice with quad injury|
|Vidi 2 Chelsea 2: Giroud spares Sarri´s side in lacklustre draw|
|Paris Saint-Germain and Marseille fixtures among latest Ligue 1 cancellations|
|Santos turn to ex-Argentina boss Sampaoli|
|Muller could miss Champions League last 16 after Tagliafico kick|
|Home hope Bekker out in front at Alfred Dunhill Championship|
|Van Bommel in touch with Robben over PSV reunion|
|Pinarello chief confident over Team Sky future|
|Stade make Ospreys tickets free in tribute to teenager|
|WRC great Loeb seals Hyundai switch|
|Madrid rumours better than sack talk, says Pochettino|
|Nadal to join Federer at 2019 Laver Cup|
|Prigoda smashes world record, Hosszu dethroned in Hangzhou|
|Former Juve CEO Marotta takes over at Inter|
|Glenn to stand down as FA boss next year|
|Muller apologises to Tagliafico for kick to the head|
|Pereira pleads for more Manchester United chances|
|Griffin hails ´phenomenal´ Lowry after Warriors demolition|
|I´ve played with Zidane and Henry, but Messi is something else – Pires|
|Dragons forward Jack de Belin charged with sexual assault|
|Durant: Raptors are the real deal|
|Cocu bids farewell to Fenerbahce after ´mutual agreement´ is made over October sacking|
|Australia unchanged for historic Perth Test|
|Champions League draw: The possible last-16 ties|
|Southee happy to be back on home soil|
|I wouldn´t swap Dembele for Neymar – Pires|
|Canelo ready to risk heavy weather against Fielding|
|Raptors upstage Warriors, Davis leads Pelicans with 44 points|
|India skipper Kohli excited by green Perth wicket|
|PSG? I haven´t decided my future yet – De Jong|
|Marcelo: Real Madrid don´t have attitude problem|
|India pair Ashwin, Rohit ruled out of second Test|
|Kovac: Ajax-Bayern draw was amazing|
|Sims re-signs with Dragons until 2022|
|Derrius Guice injury update: Redskins RB (knee) has rehab setback, undergoes 3 more procedures|
|Emery: Arsenal could sign defender in January|
|Phillies sign McCutchen to three-year deal|
|O´Neal thinks his Lakers teams could ´easily´ beat Warriors|
|A-League to add two clubs by 2020|
|Curry says he was ´obviously joking´ about moon landing comments|
|Now the beautiful part starts – Ronaldo ready for Champions League knockout stage|
|Bayern have a lot more potential – Lewandowski not satisfied despite top spot|
|Lyon star Fekir shares plaudits for Champions League progression|
|Mourinho not surprised by United´s defeat to Valencia|
|Motivation no issue for Benfica, claims Vitoria|
|Lewandowski finishes Champions League group stage as top scorer|
|Mourinho: We scored two goals and lost 2-1!|
|England have a diamond - Guardiola hails City´s Foden|
|Sterling´s carefree display invites us to love football more than others hate|
|Juventus achieved their objective, despite everything going wrong – Allegri|
|Never say never - Schmidt not ruling out All Blacks role|
|Bologna´s winning start ended in BCL|
|Di Francesco critical of Roma performance|
|Shakhtar Donetsk 1 Lyon 1: Fekir fires French side through in Group F|
|Benfica 1 AEK Athens 0: Grimaldo sends hosts out on a high|
|Young Boys 2 Juventus 1: Hoarau double seals famous victory|
|Manchester City 2 Hoffenheim 1: City through as group winners after Sane brace|