Serena Williams of the US serves to compatriot Bethanie Mattek-Sands during their third round match at the US Open Championships tennis tournament in New York, September 4, 2015. [Photo/Agencies]

"Yes, but that's between us," Venus said, breaking into a wide smile. "I think it worked."

There were moments when it appeared that Mattek-Sands, who is ranked 101st as she comes back from two hip operations, would stop Williams' streak with a varied, attacking game.

"She went for everything. She played her style, her `Bethanie Mattek-Sands tennis,"' said her husband, Justin Sands, who pointed out that Williams "is never, ever out of a match."

One issue for Williams was that she managed to convert on 3 of 16 break points through the first two sets. In the third, though, she was 3 for 5.

She also compiled a 12-1 edge in forehand winners in that decisive set.

"Usually, when I'm down, I mean, I feel like if I'm not playing well," Williams said, "I know I can take it to another level."