IT IS a puzzle Maria Sharapova just can’t figure out.

For the 18th time in a row, the Russian was outgunned by Serena Williams, yet more evidence that the American is a cut above.

Last year, they met in the final here; yesterday it was a quarter-final and as she has done in every meeting since the end of 2004, Williams came out on top, her 6-4, 6-1 victory setting up a semi-final against fourth seed Agnieszka Radwanska of Poland.

In Sharapova’s defence, Williams is a cut above the entire women’s field when she’s on song and every time she sees the Russian across the net, she invariably produces her best.

Some sportsmen and women are free to give up the ghost; boxers can fight whoever they like, top sprinters can avoid each other if they feel like it.

Sharapova does not have that luxury. Instead, if she keeps winning, especially in a grand slam event, she knows she is likely to have to face Williams at some point.

When you are good enough to have won five grand slam singles titles, it must be mentally soul-destroying to admit you are second best.

But Sharapova says she is determined to be there again.

“It's motivating because she's at a different level,” she said of Williams. “She makes you go back to the drawing board, not just for me, but for many other players. She makes you work. That's inspiring.

“I have to keep setting opportunities. Keep getting to the point where I have an opportunity to play against her. Keep finding a way to turn that around. If I don't have that chance then I don't have the opportunity to try something different.”

The kind of player who generally gives the world No.1 trouble – when she is not at her best – is one who has the variety to hit different spins and who is fast enough to make her play at least one extra ball.

By contrast, Sharapova’s hard-hit, flat groundstrokes play perfectly into Williams’ return game.

Her serve, especially since she underwent shoulder surgery in 2008, is flat, which reduces the margin for error, increasing the risk of double faults, and she struggles to get enough topspin on the ball to kick it out of the Williams hit zone.

She is not comfortable enough at the net to suddenly come in and she does not use a slice, so varying the pace is something that does not come naturally.

But what really makes Sharapova’s task impossible is the way Williams produces her best tennis almost every time they play.

Yesterday, even with the lingering effect of some minor food poisoning affecting her, Williams recovered from an early break down, edging the first set and then ripping through the second.

“I don't know what it is,” Williams said. “Something about her game. I like the way she hits the ball. Plus, when I play her, I know automatically I have to step up my game. I think that makes me play better. When I play better, when I'm forced to play better, I do well.”

Sharapova tried to serve into the Williams body and returning down the middle to cramp her space but she could not sustain it.

Next up for Williams is former Wimbledon runner-up Radwanska, who beat Carla Suarez Navarro of Spain 6-1, 6-3 in a match that was closer than the scoreline suggests.

“It will be a good match,” the top seed said. “She's been playing really well towards the end of the year, and already this year she's been very consistent.

“She presents a completely different game, an extremely exciting game. So I think it will be a long match and it will be a good match to see where I am.”