NEW YORK—Serena Williams wound up and smacked a shot directly at Dinara Safina early in the second set of their U.S. Open semifinal.

The ball hit Safina near her shoulder, ending the point, and she quickly turned her back to the net, muttering as she walked away, ignoring Williams’ attempts to apologize.

Not much later Friday, Williams tried again to say “Sorry,” except this time both players were standing at the net after the American wrapped up a 6-3, 6-2 victory. Now Williams can take aim at a third championship at Flushing Meadows and ninth Grand Slam title overall....

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