Caroline Wozniacki thrashed world number one Simona Halep to reach the WTA Finals last four in Singapore wednesday
Wozniacki, who beat Elina Svitolina in her first match, broke Halep’s serve five times in a victory that secured progress from Red Group.
The Dane is one of five players who could finish the year as world number one by winning the tournament.
Caroline Garcia won the last four games to beat Svitolina 6-7 (9-7) 6-3 7-5 in Wednesday’s other Red Group match.
The result means any one of Halep, Garcia or Svitolina could join Wozniacki in the semi-finals depending on the results of Friday’s last robin-round matches.
Karolina Pliskova has already qualified from the White Group, with Garbine Muguruza and Venus Williams playing for the second qualifying spot today.
Wozniacki, a former world number one, took the first set in 24 minutes against Halep, making only two unforced errors.
The Dane faced a break point in the fourth game of the second set, but Halep was unable to convert.
“Even if you lose the first match, you still have nothing to lose,” said Wozniacki.
“It’s just a nice cushion to have. I think that kind of motivates me to start off strong and just go for it.
“I just ran a lot of balls down and I played aggressive and I mixed up the pace. Everything I wanted to do was going my way.”
French eighth seed Garcia hit 58 winners as she fought back to condemn Svitolina to a second defeat in three days, although the Ukrainian remains in with a chance of reaching the semi-finals.
“Probably when I finish the tournament I will have questions for my team and myself, because these kind of matches I played really terrible tennis,” said Svitolina.
“OK, the girls played really, really good but the way I handled the situations in this tournament, it was like completely brainless.”