NEW YORK (AP) — There is a fascination in all sports — all walks of life, maybe — with young stars, those such as tennis player Mirra Andreeva, a Russian who is still only 18 yet already has been a Grand Slam semifinalist and is seeded No. 5 at the U.S. Open as it begins Sunday. Or João Fonseca, a Brazilian who turned 19 on Thursday and just last month became the youngest man since 2011 to reach Wimbledon's third round. Or Learner Tien, a Californian who is 19, already owns four victories over top-10 opponents and takes on Novak Djokovic at Flushing Meadows on Sunday night.

Or Vicky Mboko, a Canadian who is 18 and won the Montreal hard-court title this month while becoming the second-youngest woman to beat four Grand Slam champions at one tournament, eliminating Naomi Osaka, Coco Gauff, E

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