Angelique Kerber will drop out of the top 20 for the first time in five years after Ashleigh Barty knocked the former world number one out of the WTA Elite Trophy to reach the semi-finals along with Anastasija Sevastova and CoCo Vandeweghe.

Kerber started the year at the top of the rankings, but has plummeted spectacularly and Barty exploited the German's vulnerability to win 6-3 6-4 in Zhuhai.

While Kerber, a winner of her first two grand slam titles last year, ends a torrid season with another failure, ninth seed Barty will advance to the last four as Rose Group winner after making it two round-robin wins from as many matches.

The 21-year-old was outside the top 300 in the world this time last year but will end 2017 above Kerber after striking 35 winners and making only 12 unforced errors.

Kerber could have advanced with a straight-sets victory, but bows out having lost her opening match against Anastasia Pavlyuchenkova.

Fifth seed Sevastova goes through as Camellia Group winner following a 6-3 6-4 victory over Barbora Strycova.

Sevastova, who like Barty is making her Elite Trophy debut, followed up her win over US Open champion Sloane Stephens by breaking twice in the first set and once in the second to seal the win.

Second seed Vandeweghe also maintained her 100 per cent record to take the Bougainvillea Group, seeing off Elena Vesnina 6-3 6-2.