For the second match in a row, the No. 1-seeded Williams was hardly at her best, falling behind early and making 22 unforced errors in the first set alone. Williams needed to erase two set points for her 58th-ranked Dutch opponent in the opener, but managed to get through it.
In the final, Williams will face No. 4 Garbine Muguruza of Spain, a rematch of last year’s Wimbledon title match, which the American won.
Muguruza advanced Friday by defeating 2011 U.S. Open champion Sam Stosur 6-2, 6-4.
If Williams wins Saturday, she will equal Steffi Graf’s Open-era record of 22 major championships. Only Margaret Court, with 24, has won more.
Because of repeated rain that sabotaged the schedule this week, the women’s semifinals were played simultaneously in front of similarly empty stadiums, with Williams vs. Bertens at Court Philippe Chatrier, and Muguruza vs. Stosur at Court Suzanne Lenglen.
Both matchups featured wide disparities in age and accomplishments. Williams, 34, is a decade older than Bertens. Stosur, 32, is a decade older than Muguruza. Williams was contesting her 31st Grand Slam semifinal; Bertens her first. Stosur was in her fifth, including a run to the 2011 U.S. Open title; Muguruza her second.
Under a full cover of clouds and with the temperature in the 50s Fahrenheit (teens Celsius), Williams did not start well, not at all. Much like in her three-set quarterfinal comeback Thursday, she was not getting into position properly and kept spraying shots this way and that.
Bertens kept smacking big forehands, creating outright winners or forcing Williams into mistakes.
Williams was broken in the opening game, adding a double-fault to three errant backhands, and trailed 3-1 after 12 minutes. But it could have been worse: Bertens held three break points to go up 4-1 but failed to convert any.
Then, leading 5-3, Bertens earned a break point that was also a set point. But her forehand let her down, pushing the ball into the net on what appeared to be an easy chance, and it wasn’t the last time that stroke would falter.
Knowing that Bertens was dealing with an injured left calf — it was heavily bandaged — Williams began using drop shots to great effect. She won four points that way in the set, including one that finally produced her first break point, nearly 40 minutes in, which she cashed in to make it 5-all.
Leading 7-6 in the tiebreaker, Bertens had another chance to claim the set but again missed a forehand. Williams then drove a return winner, followed by a forehand winner punctuated with a yell, and the set was hers.
Bertens did not seem to be bothered by the way things went, and she was able to put aside that tiebreaker and get off to a top-notch start to the second set. She raced to a 2-0 lead, smacking a backhand return winner down the line to break Williams at love, then cracked a smile.
But Williams hit a forehand winner to break right back to 2-1, held to 2-all, and was on her way.
Next comes Muguruza, one of the rising stars of today’s game. Williams has won three of their previous four matches, but the lone loss came at the French Open, and it was emphatic — 6-2, 6-2 in the second round in 2014.
Williams has not lost a match at Roland Garros since.