The 36-year-old returned to the top of the rankings for the first time since 2012 after coming back from a set down to beat Haase 4-6, 6-1, 6-1.
He was handed a special prize after the match to commemorate his achievement.
Federer has enjoyed an incredible past 13 months in which has returned to the top of the sport.
He was battling with his troublesome back issues just over a year ago but has now won three of the last five Grand Slams, with Rafael Nadal winning the other two.
Much was made heading into the match against Haase at Rotterdam on Federer superseding Andre Agassi as the oldest world No 1.
And Federer took to Twitter to joke about the constant questions he faced regarding the record.
He tweeted: “Apparently I’m the oldest player with a #1ranking.
“Somebody might have mentioned that to me already but I hard a hard time hearing.”
“It’s been a long road, and sometimes windy, but feels surreal to be back at the top.
“I’m just happy to be healthy and playing tennis every day.”
Federer will now face Andreas Sappi in the semi-final later today with the winner facing either David Goffin or Grigor Dimitrov in the final.
Source : EXPRESS