(CNN) Once again, history is on the line for Serena Williams at the US Open.
But, as Williams is trying to protect her No. 1 ranking and chase an Open era record 23rd grand slam title, the final tennis major of 2016 could also provide the perfect setting for a new generation of American players eager to be cast as her potential successor.
With the 34-year-old Williams no longer dominating, and struggling with a shoulder injury that derailed her Olympics campaign, 21-year-old Madison Keys sees her as an inspiration.
"The biggest thing that I'm always very impressed with is her fire, she's No. 1 in the world, she's won all these grand slams, but she always wants more," Keys told CNN ahead of the New York tournament, where she will be the eighth seed when the two-week event starts Monday.
"I don't think very many people have that characteristic, and seeing that so much is probably one of the biggest reasons why she's done what she's done," said Keys, who just like Williams has a hugely powerful serve and aggressive game style.
If the past 12 months are anything to go by, we could be in for another surprise in New York.
After Roberta Vinci pulled one of the greatest upsets of all time by ending Williams' historic quest to win the first "calendar slam" since 1988 in last September's semifinals, the US Open was won by fellow Italian Flavia Pennetta, who promptly announced her retirement.
Williams then lost in January's Australian Open final to Germany's Angelique Kerber -- who will be guaranteed the No. 1 ranking if she wins the title on September 10 -- while Spain's Garbine Muguruza outplayed her in the final at Roland Garros.
Although Williams righted the ship with a win over Kerber in July's Wimbledon final, her reign as Olympic champion was ended by Ukrainian Elina Svitolina in the third round at Rio 2016 -- a tournament won by Puerto Rican outsider Monica Puig.
Williams, who turns 35 next month, then pulled out of this month's US Open warmup event in Cincinnati, citing a shoulder injury.
She will begin her bid for history against Russian Ekaterina Makarova in a rematch of their 2014 semifinal, which Williams won before lifting the US Open trophy for the sixth time, equaling Evert's Open-era record.
Her older sister Venus -- one of Keys' childhood idols -- is seeded sixth. The two-time US Open champ -- a quarterfinalist last year -- faces Ukraine's Kateryna Kozlova first up.
Keys is in the other half of the draw to the Williams duo, and faces fellow American Alison Riske in the first round.
Although she has always had immense talent, Keys believes she has finally found the right mindset to be a contender for a grand slam title.
Ever since she broke through in 2015, reaching the Australian Open semifinals and Wimbledon quarterfinals, Keys has been widely touted as "the next Serena."
Williams declared herself " a fan" of Keys after beating her in last year's Melbourne semis, saying: "It was an honor for me to play someone who will be ranked No. 1 in the future."
Read more: Graf - I want Serena to break my record
But thanks to Thomas Hogstedt, a Swede who turned Maria Sharapova into a French Open champion, Keys has found the right consistency to become a top-10 tennis player.
"He's really helped me mostly on things that are on the practice court -- having a very focused and intense practice where there is really no lapses in focus," Keys, who has reached the fourth round in all three slams this year, told CNN.
"My practice may only be one hour, 15 minutes, but it's very high intensity, very focused and we get a lot done. That is shifting over to my match play, where I feel I used to have two or three games of bad decisions, or loss of focus. And now, instead of three games, it's maybe two points."
At the age of nine, the Rock Island, Illinois native moved to Florida with her family to train at the academy run by 18-time grand slam winner Chris Evert and her brother John, who became her coach.
After winning several prestigious junior events, Keys turned pro in 2009 and became the youngest player since Martina Hingis in 1994 to win a main draw WTA match at an event in Ponte Vedra Beach, Florida aged 14. By 2013, she made the top 50.
Keys was guided by former Wimbledon winner Lindsay Davenport during her breakout season in 2015. After brief spells with former U.S. tennis pro Jesse Levine and former top-ranked Mats Wilander at the start of 2016, Keys started working with Hogstedt in April. Having struggled with injuries in the past, she now also travels with renowned fitness trainer Scott Byrnes.
The changes have paid off.
Known for an aggressive playing style built around one of the most powerful serves in the women's game, Keys won a grass-court event in Birmingham, England in June to become the first American woman to reach the top 10 of the women's tour since Serena Williams in 1999. At 21, she's also the youngest member of the elite group.
Who will win the US Open? Have your say on our Facebook page
Other highlights included reaching the final on clay in Rome, where she lost to Serena, and on hard court in Montreal, where she was beaten by former No. 2 Simona Halep. At the Rio Olympics, Keys played for the bronze medal.
Although she lost out to two-time Wimbledon winner Petra Kvitova, Keys called her first Olympic Games in Rio de Janeiro "one of the coolest experiences of my life."
In between her tennis commitments, she watched the swimming and saw Simone Biles excel in the gymnastics arena.
Chris Evert has been so impressed with Keys' progress that she tipped her as one of the main challengers for Williams alongside more established names such as second-ranked Kerber, third-ranked Muguruza and No. 5 Halep, a semifinalist in New York last year.
"You really have to make that mental and emotional commitment to the game," Evert said in a conference call for tennis writers organized by U.S. broadcaster ESPN this week. "I think that's what Madison Keys is learning right now. She's made more of a commitment to tennis. She could still be better."
Visit CNN's Open Court page for more tennis news
If Keys keeps the right mindset, Evert adds, anything is possible.
"I'm a Madison Keys fan because of her power on her serve and her ground strokes," she said. "And if she could ever get it all together and believe and trust herself and play her 'A Game,' she could be a threat."