Third seed Karolina Pliskova thrashed Wimbledon champion Garbine Muguruza for the loss of just four games at the WTA Finals to become the first player to qualify for the semi-finals.
The Czech, 25, won 6-2 6-2 in just 62 minutes to ensure she will win the White Group, with one match remaining.
Venus Williams earlier overcame French Open champion Jelena Ostapenko 7-5 6-7 (3-7) 7-5 in three hours 13 minutes.
Muguruza and Williams will play for the second qualifying place on Thursday.
World number one Simona Halep plays sixth seed Caroline Wozniacki in the Red Group with a semi-final place on the line at 12:30 BST on Wednesday, before fourth seed Elina Svitolina and eighth seed Caroline Garcia try to keep their hopes alive.
Pliskova powers through to last four
A second 6-2 6-2 win in three days, after she beat Venus Williams by the same score on Monday, carried Pliskova into the semi-finals in impressive fashion.
The Czech could have won even more quickly as her power left Muguruza struggling to keep pace, but the Spaniard made a brief fightback from 6-2 5-0 down before succumbing soon after.
"I'm just happy that I don't have to play the next match for qualifying, because last year I was playing for it and it was pretty stressful," the Czech told the WTA website.
"I'm always happy if I can win few matches in a row, it gives me always extra confidence.
"There is one more match to go in the group, which I want to try to enjoy, not to lose, but just to try to enjoy the tennis a little bit more."
Muguruza, who has now lost seven of her nine meetings with Pliskova, said: "I'm not going to think a lot about this match, because it's one of those days when, you know, the other opponent is playing good and your tennis is not at the level it should be to play against this type of player."
Williams wins three-hour Singapore epic
Williams, 37, took 11 of 20 breaks of serve to win her second round-robin match in the White Group, having lost her opener against Pliskova on Sunday.
"A win is a win, that's all I can say," said Williams. "Maybe I got a little luck. Who knows? But I'm on to the next match now."
The American, twice a Grand Slam finalist in 2017, missed a match point in the second set but came out on top in the 69-minute third set, which included six consecutive breaks.
"I was just a little bit tight in some moments," said Ostapenko, who cannot now reach the semi-finals. "When I had to serve, I didn't really serve."
|WTA Finals line-up|
|RED GROUP||WHITE GROUP|
|1. Simona Halep (Rom)||1. Garbine Muguruza (Spa)|
|2. Elina Svitolina (Ukr)||2. Karolina Pliskova (Cze)|
|3. Caroline Wozniacki (Den)||3. Venus Williams (USA)|
|4. Caroline Garcia (Fra)||4. Jelena Ostapenko (Lat)|