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During one point midway through the second set, Vondrousova hit an underspin, scooped forehand approach shot that landed right on the line - prompting Osaka to stare at the line for a few seconds in apparent disbelief.

Her preference for a classic, underspin backhand was a nod to former greats, but a whiplash forehand and heavy serving, were tailor-made for the emerging power game.

It is far superior for slice, or underspin; it is also much better for drop shots and volleys and for retrieving difficult-to-reach balls.

From the first point on, Pennetta pinned Vinci deep in the court, forcing her to hit underspin backhands repeatedly.

But until now, research on the topic has tended to focus on topspin or underspin — movement perpendicular to the direction in which the ball is moving.