Player growth and development usually occurs slowly over multiple seasons, but in the case of Stefanos Tsitsipas and Aryna Sabalenka, they've come of age before our very eyes this US Open Series.

Tsitsipas has proved he is a major force to be reckoned with right now by learning and improving with each match. After putting together a solid week to reach the semifinals in Washington D.C., the Greek struggled against eventual champion Alexander Zverev, falling in straight sets.

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But the young 20-year-old lives and learns, quickly. He turned around and put together the week of his life in Toronto, beating four Top-10 players en route to his first ATP Masters 1000 final.

The new world No. 15 saved two match points against Zverev just one week after his defeat to the German, and also survived an epic against Wimbledon finalist Kevin Anderson.

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World No. 1 Rafael Nadal proved too big a hurdle for Tsitsipas. He also fell early in Cincinnati, understandably running out of steam after two weeks of highs.

On the women's side, fellow 20-year-old Sabalenka has shot up the rankings to a high of No. 25 behind her big-hitting game. She made her big-stage debut last fall in the Fed Cup final for Belarus, and now she's picking up steam in 2018 by pulling off Top-10 wins left and right.

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Earlier this summer in Eastbourne, the Belarusian reeled off several tight three-set wins against top opposition to book her spot in the WTA Premier final against Caroline Wozniacki. Despite having multiple set points in both sets, Sabalenka lost.

In Montreal, she avenged her loss to Wozniacki by learning how to play the big points better, this time saving multiple match points with daring play to reach the third round.

The following week, Sabalenka continued to develop rapidly in Cincinnati. She racked up three-set wins over Johanna Konta, Karolina Pliskova and Caroline Garcia, before knocking out Madison Keys to reach the semifinals.

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She would lose to world No. 1 Simona Halep in competitive straight sets, and is playing in New Haven this week, where she's already knocked out former US Open champion Sam Stosur.

It's pretty clear, Sabalenka has arrived. Both she and Tsitsipas will be biggest young stars to watch when the US Open starts next week.