Zhuhai - Sloane Stephens retired in her match against Barbora Strycova at the WTA Elite Trophy on Friday, extending her winless streak since lifting the US Open trophy in September.
The American, part of the US team looking ahead to next weekend's Fed Cup final against Belarus, crashed out of the tournament in Zhuhai with a knee injury that sealed her defeat to Czechoslovakia's Strycova 5-0.
"My knee wasn't doing what I wanted it to do," she told journalists after the match.
"So, yeah, it's like it's better to just stop now, because obviously I wasn't moving too quickly out there. I just thought, yeah, better if I just stop."
She added that she hoped to be fit enough for the Fed Cup.
"Obviously I've got to get my knee in order first. Hopefully I can do that before next weekend."
Earlier Julia Goerges beat top seed Kristina Mladenovic to reach the semi-finals alongside American CoCo Vandeweghe, Australia's Ashleigh Barty and Anastasija Sevastova of Latvia.
Twenty-four-year-old Stephens, whose fairytale win at Flushing Meadows saw her world ranking shoot up, has been on a losing streak recently, tumbling out of both the China Open and Wuhan Open before pulling out of Hong Kong.
Her turbulent run meant she failed to make the WTA Finals in Singapore, while she was left out of the semi-finals line up in Zhuhai after a loss to Latvian Anastasija Sevastova.
She has blamed her poor stretch on having less time to practise since winning in New York.
"I think that playing tournaments when you're not ready is probably the worst possible thing you can do, and I think the last tournaments I have played, last three tournaments, have been a good example of that."
She said at a press conference earlier in the week it had been "very tough" playing straight after her career-high win in the US but she "didn't want to just bail".
Stephens is not the only player to reference the gruelling schedule, with Vandeweghe also chiming in earlier this week.
"I was going to miss most of China... just because I was exhausted after the US Open. Emotionally I didn't want to play any tennis," Vandeweghe told reporters.
"For people that don't ever experience it, it's very difficult to come back emotionally, more so than physically, after a Grand Slam."
But she added with the China Open, the fine was too heavy not to play.
"I showed up in Beijing because it cost less to fly there and play than actually it was to get fined."
In another match Friday China's top player, Peng Shuai, downed Elena Vesnina 6-2, 1-0 after the Russian was also forced to retire.
But despite her win, the round robin format meant Peng, a wildcard entry, could not go through to the semis.
The 25-year-old, the oldest player in the tournament, lost to Vandeweghe earlier in the week.
Result from the fourth day of the WTA Elite Trophy tournament in Zhuhai, China, on Friday (x denotes seeding):
Julia Goerges (GER x7) bt Kristina Mladenovic (FRA x1) 6-2, 7-6 (7/4)
Shuai Peng (CHN x12) bt Elena Vesnina (RUS x6) 6-2, 1-0 retired
Barbora Strycova (CZE x11) bt Sloane Stephens (USA x3) 5-0 retired