In Sunday's final, the 36-year-old Federer will win his 97th career title if he beats Grigor Dimitrov, dubbed "Baby Fed" early in his career for his style similarities with the Swiss great.
Federer would become the oldest top-ranked player in ATP history, beating Andre Agassi by three years.
-Federer won a record third Rotterdam title, having previously triumphed in 2005, defeating his current coach Ivan Ljubicic, and in 2012 with victory over Juan Martin del Potro.
Federer has won both the tournaments he has played this year, which also includes January's Australian Open - for the sixth time.
It was the first time the Swiss great reached the position since 2012, and comes nearly 14 years after he first became World No 1.
Federer now holds a 345-point lead over Nadal and will make a decision soon whether to take part in the Dubai Open that takes place from 26 February.
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"Thank you very much for the great, great welcome I got this week". In between it was a battle, it was nerve-wracking getting back to No. 1, but I was able to manage my nerves and the expectations.
It was a great week, but I just came up short today.
"I started finding energy midway through the first set, but the start was tough".
"Grigor is a great player and a great athlete and he's been playing super well in recent months", he said. Anything can happen out there, all credit to Roger.
"Reaching No. 1 is one of, if not the ultimate achievement in our sport", Federer said.
Haase made clear he was going to make Federer fight, and broke him in the ninth game and fired an ace to take the set 6-4.