Teenagers once routinely won Grand Slam tournaments, especially on the women’s circuit, currently dominated by the soon-to-be-34-year-old Serena Williams. And while Nadal — who won the French Open at 19 in 2005 — believes that the process is cyclical and that a teenager will inevitably rule again, it still seems more likely that a young female player could break through before a male equivalent can win seven grueling best-of-five-sets matches in a men’s Grand Slam draw.

But the women’s tour, which restricts the number of tournaments a teenager can play between the ages of 15 and 18, is far less kid-friendly than it used to be.

At the same 2014 Wimbledon tournament in which Kyrgios announced himself, Eugenie Bouchard was a finalist after reaching semifinals in two previous Grand Slam events that year. She was suddenly a 20-year-old magazine cover girl, heralded as the sport’s newest marketing sensation, its next Maria Sharapova.

This year, she has lost her first match of most tournaments she has entered, tumbled out of the top 10 to No. 25, has had three coaches since losing that Wimbledon final, and this week has been working with Jimmy Connors in New York. What happened? Bouchard has been at a loss to explain it, other than saying in Toronto earlier this month that a nagging abdominal injury had taken its toll. She said she was healthy again but her results did not improve — she absorbed a 6-0, 6-1 drubbing by Roberta Vinci at the Connecticut Open.

Her rapid descent, while mysteriously swift, did not come without hints of the attendant pressures and temptations of fame. She was said to be increasingly difficult for her longtime coach, Nick Saviano, to deal with. She spoke openly about her brand after signing lucrative endorsement deals. Canadian reporters who follow her have suggested that she seemed to have lost weight and possibly some muscle tone.

In Toronto, Bouchard lost her first match to 18-year-old Belinda Bencic, who proceeded to upset Serena Williams in the semifinals and win the tournament. Bencic, from Switzerland, is a quasi-protégée of Martina Hingis and Hingis’s mother, Melanie Molitor. Hingis, who at 16 was ranked No. 1 and has long disagreed with age restrictions, said Bencic could be the one to take women’s teenage tennis back to the future.