Switzerland's Roger Federer plays a forehand return to Netherlands Robin Haase during their quarter-final singles tennis match for the ABN AMRO World Tennis Tournament in Rotterdam on February 16, 2018
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The Swiss will play world number five Grigor Dimitrov in Sunday's final.
Federer, who has won the title twice before, was not at his fluent best but was seldom troubled after breaking the Italian early in the first set and proved too good on all the big points.
The 36-year-old guaranteed his return to the top of the world rankings on Friday after more than five years, becoming the oldest man ever to reach the summit.
In the other semi-final, Belgium's David Goffin was forced to retire against Dimitrov after suffering a freak eye injury.
The 27-year-old was trailing 6-3 0-1 when he framed an attempted volley directly into his left eye and was unable to continue. It was not clear how serious the injury was.
Pieter van den Hoogenband
Stages 19 & 21