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Yellen is due to deliver her speech in Chicago after midday. Other US central bankers have signaled in recent days that the US central bank could lift interest rates later this month.
Other Fed speakers were also due at public appearances Friday, including Vice Chairman Stanley Fischer, who will speak in New York.
About 10 minutes into trading, the Dow Jones Industrial Average was at 21,027.60, up 0.1 percent.
The broad-based S&P 500 was flat at 2,382.02, while the tech-rich Nasdaq Composite Index added less than 0.1 percent at 5,862.99.
Costco Wholesale dropped 3.8 percent after reporting second-quarter profits fell 5.7 percent to $515 million. The big-box retailer said it was hiking annual membership rates.