Three-time major champ now a big doubt for the US Open due to ongoing hip injury.
The Spaniard jokes "leave something for the young ones" as Federer prepares to return.
Federer begins his hard-court campaign in Montreal ahead of the US Open.
Two-year rankings would allow more players to take breaks and rest up.
Nadal can become world No1 if he makes the semi-finals at the Rogers Cup this week.
Nadal is on course to regain his world number one ranking at the Rogers Cup.
The legendary pair continue to roll back the years and are the top two seeds for the Rogers Cup.
The Swiss number two is "extremely disappointed" about not being able to defend title at Flushing Meadows.
Nadal could become the new world number one with a win in the Rogers Cup.
Federer will be looking for his sixth title of the year ahead of the US Open.
Federer won his 19th men's singles Grand Slam title at Wimbledon this year.
Thiem is one of many young players who are expected to be the new generation.
Injuries and poor form have disrupted Murray's year so far.
Nadal is just 285 points behind world number one Andy Murray.
Nadal has won four titles in the calendar year so far, including a 10th French Open title.