Thrashed in the first set and trailing at the second, Kiki Bertens feared she was out of her depth before beating Angelique Kerber.
Sloane Stephens got the better of Naomi Osaka on Monday, but the US Open champion was looking to stay positive.
A seemingly comfortable evening for Angelique Kerber was thrown off course by Kiki Bertens' spectacular fightback in Singapore.
In a clash of two debutants at the WTA Finals, Sloane Stephens overcame Naomi Osaka to record a win in the Red Group.
Caroline Wozniacki was defeated by Karolina Pliskova at the WTA Finals on Sunday, with the latter relishing her busy schedule.
Elina Svitolina is determined to go from strength to strength in the WTA Finals and proving she deserves to be among the elite.
John Isner says the All England Tennis Club should name the final set tie-breakers at Wimbledon after him.
Caroline Wozniacki beat Karolina Pliskova in the last four in Singapore last year, but it was a different story in their White Group opener.
Elina Svitolina hit the ground running in Singapore, beating Petra Kvitova for the first time in four years.
Belinda Bencic fell just short in the Luxembourg Open final as Julia Goerges took home the trophy.