Naomi Osaka is on the brink of elimination from the WTA Finals after her struggles against Angelique Kerber continued in Singapore.
The US Open champion, beaten by the previous holder of that title, Sloane Stephens, in her Red Group opener, went into Wednesday's contest having lost her last three meetings with the Wimbledon champion.
And Kerber made it four in a row over the Japanese, pouncing on a frenetic Osaka display in which she produced 50 unforced errors to claim a 6-4 5-7 6-4 triumph.
Germany's Kerber now faces a decisive encounter with American number one Stephens, whose match with Kiki Bertens will see the Dutch star through if she triumphs.
The match started with back-to-back breaks to love before Kerber forged ahead in the third game when Osaka clattered a shot into the net, but she came under severe pressure when attempting to serve out the set.
Osaka saw five break-back points go begging and Kerber subsequently unleashed a booming superb serve to take the lead.
A backhand return winner on Kerber's second serve saw Osaka go 2-0 ahead in the second but that proved only to be the first of four breaks as she failed to capitalise while seemingly bothered by a hamstring issue.
Her chances of progressing to the semi-finals appeared all but over when a double fault handed Kerber a 5-4 lead and the chance to serve out the match, but Osaka responded by stringing three successive games together, bringing up set point with a backhand winner before Kerber fired into the net.
The third proved a much tighter affair, and the exasperated Osaka proved the first to crack courtesy of some brilliant Kerber defence and a fine forehand pass her 21-year-old opponent had no answer to.
Osaka let opportunities to hit back slip as a string of return errors allowed Kerber to hold from 40-15 down and her victory was sealed, fittingly, with another Osaka error when she smashed into the net with the court at her mercy.
Kerber  bt Osaka  6–4, 5–7, 6–4
Kerber - 24/30
Osaka - 42/50
Kerber - 1/6
Osaka - 6/2
BREAK POINTS WON
Kerber - 6/9
Osaka - 5/18
FIRST SERVE PERCENTAGE
Kerber - 66
Osaka - 50
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Kerber - 59/51
Osaka - 70/38
Kerber - 109
Osaka - 104