Billionaire investor Ken Griffin’s flagship hedge fund rallied last month when the broader market was rattled by tight monetary policy as well as rising recession fears, according to a person familiar with the returns.

Citadel’s multistrategy flagship Wellington fund gained 1.7% in September, bringing its 2023 performance to 12.6%, the person said. The S&P 500 pulled back 4.9% last month, suffering its worst month of the year. The equity benchmark is still up 11% for the year.

The market has grown more volatile and fragile as investors grapple with a higher-for-longer interest rate regime. Stocks resumed the sell-off this week as the 10-year Treasury yield surged to a 16-year high. Many notable investors, including Pershing Square’s Bill Ackman, have warned of a deteriorating economy after a series of aggressive rate hikes.

Griffin, founder and CEO of Citadel, told CNBC last month he was skeptical that this year’s rally, powered mostly by artificial intelligence-related stocks, can be sustainable.

“I’m a bit anxious that this rally can continue,” Griffin said. “Obviously one of the big drivers of the rally has been … just the frenzy over generative AI, which has powered many Big Tech stocks. … We’re sort of in the seventh or eighth inning of this rally.”

Citadel’s equities fund, which uses a long/short strategy, was up 1.1% in September and 10.7% this year, while its global fixed income fund is 8.8% higher so far in 2023, the person said.

Citadel had $61 billion in assets under management as of Sept. 1. The Wellington fund soared 38% in 2022 for its best year ever.