Defending champion Williams, who won the first of her three Roland Garros crowns in 2002, started slowly for the second day running, her early play littered with the unforced errors that had characterised Thursday's laboured three-set quarter-final win against Kazakh Yulia Putintseva.
The champion was broken in the first game by her unseeded opponent who, entering the contest on a 12-match winning singles streak in all competitions, showed no sign of nerves in her first major semi-final.
Williams eventually took the first set 7-6 on a tiebreak that she edged 9-7, settling into a more comfortable rhythm in the second set, which she won 6-4.
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"The first set was not very easy but I think that today I played better. (Bertens) played very well," Williams said courtside.
For 58th-ranked Bertens, Friday's defeat marked the end of an unprecedented major run she dubbed "crazy" after her quarter-final win on Thursday over Swiss eighth seed Timea Bacsinszky.
But she may well rue a failure to take her chances on Friday, when she converted just two of 10 break points.
After breaking early on Bertens -- mixing the occasional dropshot in with powerful groundstrokes, especially on the forehand, and occasionally outhitting the American -- had three chances to break again in the fifth game for what would have been a 4-1 first set lead.
But Williams held and, after saving a set point at 3-5, broke back in the next game with the help of a net chord that, had it dropped back on the American's side, would have given Bertens two further set points.
Williams closed out the 57-minute set by bludgeoning a forehand down the line, set up by a powerful serve to end a set that took just under an hour.
With the Philippe Chatrier showcourt still little more than
half full as the unseasonably cold weather kept thousands of ticket-holders away, the drama continued into the second set, when Williams was broken again to trail 2-0.
But she broke straight back and conceded just two further games, closing out the match 6-4 on her fifth match point, when Bertens hit a forehand long, to reach Saturday's final.
There she will attempt to equal Steffi Graf's professional-era record of 22 grand slam singles titles.
(Reporting by John Stonestreet, Editin gby Martyn Herman)
