That is, of course, a tall order, but McNally showed plenty of poise and potential under the closed roof in Ashe Stadium after reaching the semifinals at the WTA event in Washington this summer.

A powerful athlete, McNally likes to serve and volley, a tactic now rare enough to elicit gasps from a savvy tennis crowd. Like Federer, she has a particularly crisp one-handed slice backhand, and unlike Federer, she also can drive her backhand with two hands. She used that two-hander to try another rare ploy on Wednesday: attacking Williams’s second serve immediately after the bounce and following the return to net.

The results were mixed, but the approach was destabilizing.

“It was definitely something to get used to,” Williams said of McNally’s style. “You don’t play players like her who have such full games. I just think she over all played really well.”

Mixing speeds and spins and coming up with big serves when she needed them, McNally applied and absorbed pressure and was able to convert her only break point of the match to take a 6-5 lead in the first set.

She won it by rallying from 0-40 to hold serve, waving her arms to pump up the crowd as she arrived at her seat with a swagger in her step.

Seemingly rattled by McNally’s shifting tactics and forays to net, Williams was far from her sharpest in the opening set, making 15 unforced errors.