Roger Federer said Novak Djokovic would not have a chance to become number one at the end of the year if he defeated him at Wimbledon.
Federer moved to the ATP Finals semifinal after producing an excellent display at the O2 Arena over Djokovic.
The Swiss served superbly, only dropping four points on his second service and winning 78 percent on his service to frustrate his opponent.
And a victory for Federer means Nadal ended the year as world number 1 and number 1 at the end of the year.
The 38-year-old can also take revenge on Djokovic after losing to him at Wimbledon in a heartbreaking way in July.
After the match, Federer was asked about helping Nadal reach the top spot and said Djokovic was just as close as he was because of what happened at SW19.
He said: "I know there are many things for Novak today. If he does not win Wimbledon, maybe he will not even achieve that, to be honest.
"That's what I told myself too. Because he beat me at Wimbledon, Novak was actually as close as he was.
"And Rafa also had an unrealistic stretch. Both had great years.
"I'm glad I can beat both of them this year, but they both deserve a higher ranking than me this year.
"But, yes, it was a good day for tennis. I'm sure to some extent, not because I defeated Novak but the World No. 1 race was riding the match, too, and the atmosphere in the building was very very good.
"I was able to produce under pressure, and I tried to remind myself to play with less pressure than Novak had throughout the game, and it worked.
"I'm very happy. I'm very happy."
Federer joins Dominic Thiem to advance to the semi-finals of the Bjorn Borg group.
But Nadal still faces a difficult task to qualify for the next round with a match to be won against Stefanos Tsitsipas.
However, even a victory for the Spaniard would not be enough if Alexander Zverev defeated Daniil Medvedev because Germany won a head-to-head match with Nadal.
Tsitsipas has booked his place in the semifinals after defeating Medvedev and Zverev.