Roger Federer celebrated his soon to be confirmed return to world No.1 with a routine win over Andreas Seppi that booked a Rotterdam Open final against Grigor Dimitrov.

The 36-year-old Federer will become the oldest male player to scale tennis's heights when the rankings are updated, having secured a comeback win over Robin Haase in Saturday's (AEDT) quarter-final.

The 20-time grand slam champion followed that up by seeing off Seppi 6-3 7-6 (7-3) in the last four on Sunday (AEDT), the Swiss maestro not always at his best but never truly troubled by a spirited opponent.

He will now aim for a third title in Rotterdam against Dimitrov, who progressed after David Goffin was forced to retire from the other semi-final with a nasty eye injury.

Federer had to save a couple of break points in a scratchy opening, but a big forehand that Seppi could only put into the net and a skewed backhand from the Italian presented the first break to the Swiss.

More uncharacteristically sloppy serving led to Seppi breaking straight back when Federer went long from the baseline.

From there, though, the veteran shifted through the gears, a forehand winner in game seven restoring his break lead and another thumping groundstroke, after some typically graceful play, earning a third break that sealed the opening set.

The second was a much more competitive affair, with neither player offering a sniff on serve.

That meant a tie-break was required and Seppi started by lashing a forehand to earn the first mini-break.

Federer reached into his very top draw to quickly turn the breaker on its head, though, winning the next five points.

Seppi kept the match alive by breaking Federer's next two serve points, but it was not enough to shift the momentum of the contest as the new number one converted his first match point to ensure he will have the chance to celebrate his landmark achievement with a title on Monday (AEDT).