Roger Federer won his 97 career title on Sunday 6-2, 6-2 over second seed Grigor Dimitrov of Bulgaria to win the Rotterdam tournament.
The 36-year-old Swiss plays Italy's Andreas Seppi in the second semi-final.
In the Open era, only American Jimmy Connors has won more titles with 109.
Federer would surpass American Andre Agassi as the oldest World No.1 if he wins his next match in Rotterdam and he would also set another record of the longest gap (five years and 106 days) between periods as No. 1.
"It's unbelievable to be number one again after all these (six) years - this is one of the best weeks of my life".
"I had a great first match and a great last match". World No. 5 Dimitrov had a 6-3 0-1 lead when the fourth-seeded Belgian was forced to quit the match. Federer chose to play in Rotterdam rather than in Dubai and the decision served him well as he won in the Dutch city for a record third time, after previously winning in 2005, defeating his current coach Ivan Ljubicic, and in 2012 with victory over Juan Martin del Potro. "Grigor is a great player and a great athlete and he's been playing super well in recent months".
The Bulgarian duplicated his 2013 semis at the Ahoy Stadium with a win in an hour and a quarter, sending over a dozen aces and saving both break points he faced against his 20-year-old opponent.
"I was able to improve my tennis today when I needed to, I was able to take advantage".