Federer returned from a long injury lay-off in January last year to experience a career resurgence as he won the 2017 Australian Open in what was his first Grand Slam win since 2012.
The Swiss ace would go on to win another Tour-leading six titles, including a record-breaking eighth Wimbledon crown to further cement himself as arguably the greatest player of all time.
He was not able to cap off a remarkable year as the world number one, though, as his withdrawal from the Paris Masters allowed Nadal — who had his own career resurgence with six titles and two Grand Slams — to end the year at the top of the ATP tennis rankings for the first time since 2013.
However, with Federer's splendid run continuing in 2018 after yet another Australian Open victory, the 36-year-old is just 155 points behind Nadal in second.
Having entered the Rotterdam Open via a wild card which begins on Monday (12 February), he could become the oldest world number one of all time if he reaches the semi-final, overtaking Andre Agassi's record at 33 years of age.
In addition, Federer can break Nadal's record for the longest time between first becoming a world number one and the last time they held it, according to the Express.
Nadal first reached the summit in August 2008 and if he drops to second in the rankings, he will see his record stand at nine-and-a-half years.
Federer became the world number one for the first time in February 2004 and reaching the last four in Rotterdam will see him break Nadal's record with 14 years.
It will not be smooth sailing for the 20-time Grand Slam champion, though, as he could face his compatriot in Stan Wawrinka in the quarter-final stage.
Federer will face Ruben Bemelmans in his opening match on Wednesday (14 February).