This is not the first time Nadal has struggled with a knee injury in his career but he remains optimistic about playing in the ATP finals in London. In the second set, Nadal showed his dominance and won the second set by 6-4.
The 31-year-old added, "It's important to play tennis as long as possible". It has been an incredible year.
"I think Roger does still care about being No 1 but to get there he's got to be smart about it". By becoming - at the age of 31 - the oldest player in the game's history to achieve this spectacular feat, Rafa has once again demonstrated his unwavering work ethic, determination and passion for the game.
Nearly nothing could separate the rivals in the shortest and longest points but Nadal had a slight advantage in the mid-range rallies, despite hitting a lot of unforced errors. He continues to re-write the record books and is a credit to our sport.
The injury has thrown his participation at the World Tour Finals at London's O2 Arena, scheduled to run from November 12 to 19, into doubt.
Krajinović has progressed to the last four by virtue of Nadal's retirement. While in Australian Open he was defeated by rival Federer, in the next major tournament he claimed a historic 10th French Open title against Stan Wawrinka.
After wrist problems badly affected his 2016 season, Nadal had stayed fit throughout a remarkably successful 2017 campaign, winning two grand slam titles and returning to the top of the rankings.
Nadal played down the issue after the match but withdrew ahead of his scheduled quarter-final against Filip Krajinovic.