In the other semi, second-seeded Jelena Jankovic will meet two-time champion Amelie Mauresmo.
Loit, only the second qualifier in tournament history to make the quarterfinals, broke Williams when she was serving for the first set at 5-3. Williams threw her racket on the ground but broke right back and had few other problems.
"Definitely a tricky match,'' Williams, the Australian Open champion, said. "Definitely good to get through, you know, I don't play too many lefties.
"I definitely don't feel at my best,'' Williams added, "but I feel like I'm getting there.''
Dementieva gave Williams her only loss of the year in the Sydney semifinals, but Williams avenged that in the Australian Open semifinals en route to her 10th Grand Slam singles title.
Third-seeded Dementieva, already a two-time winner on tour this year, beat French wild card Nathalie Dechy 6-3, 6-2 without conceding a single break point despite landing only half of her first serves.
"Against someone like Nathalie, you have to be very focused not to give any chance,'' Olympic champion Dementieva said. "I was trying to be aggressive, dictating the points from the very beginning.''
Beating Williams on Saturday would mark Dementieva's 500th career victory.
Mauresmo made sure France will be represented in the semifinals for a 12th straight year when she routed fourth-seeded Agnieszka Radwanska of Poland 6-2, 6-0.
Mauresmo was in sight of her sixth Open GDF Suez final - she won twice in 2001 and '06.
Jankovic, entered as a wild card rallied past fifth-seeded Alize Cornet of France 5-7, 6-4, 6-4 in a match with wild momentum swings. Both players struggled with their service games as Cornet dropped serve 10 times and Jankovic nine times.
"It was an intense match,'' Jankovic said. "It was very tough for me physically, especially at the end of the third set. It was the one who could stay longer, who had more endurance, who could finish those points off.''
Jankovic blew a 5-2 lead in the first set.
In the third set, the Serb rallied from 3-1 down, winning four straight games for a 5-3 lead before clinching the match on a backhand return shanked by Cornet.
During the changeover at 4-3 in the third, Jankovic had a trainer change bandages around her feet.
"I have little blisters underneath my feet,'' Jankovic said. "Because we played so long, my shoes and socks were completely wet. So the bandages moved and I started to have pain in my feet.''
Cornet was not spared from pain either. She suffered from an inflammation of her right shoulder tendon last week and took anti-inflammatory drugs until Monday. But she apparently hasn't recovered yet.
"I felt the pain in my shoulder in the beginning of the third set,'' Cornet said. "It was almost impossible to serve well. From the moment I wasn't serving well enough against a player ranked third, I was under pressure.''
Williams, Jankovic make semis at Open GDF Suez
Williams, Jankovic make semis at Open GDF Suez
Camacho Satisfied With Osasuna’s Point At Deportivo La Coruna
