Angelique Kerber, Serena Williams

Angelique Kerber made her U.S. Open debut in 2007 as a 19-year-old. The big-swinging, left-handed German's first opponent was two-time champion Serena Williams at Arthur Ashe Stadium. Williams handled Kerber, 6-3, 7-5, and they have not met at the U.S. Open since, but if they do face off this year, it will be in the final as the No. 1 and No. 2 players in the world, respectively.

Obviously, matches need to be played in the meantime, but as the tournament inches toward its final weekend -- Kerber won her quarterfinal Tuesday against 2015 runner-up Roberta Vinci, 7-5,6-0 -- the stars are aligning for a Williams-Kerber title match.

And that would be good for the sport, considering Kerber, now 28, is Williams' first legitimate rival in nearly a decade.

Angelique Kerber, Serena Williams

Garbiñe Muguruza beat Williams in the French Open final. Vinci upset her in last year's U.S. Open semifinals. Sloane Stephens took down Williams in the Australian Open quarterfinals three years ago. Samantha Stosur beat her in the 2011 U.S. Open final.

But those were all one-time deals.

What about Maria Sharapova, Victoria Azarenka, Agnieszka Radwanska and Caroline Wozniacki, who have all been among the top players in the world for the last half decade? Nope. None of these players has ever given Williams consistent adversity. Williams has not had consistent rivals since Justin Henin and sister Venus were in their primes. From 2003-2007, Henin was 5-2 against Williams. Through 2008, Venus was 8-10 versus Serena (she is 3-6 since).

So what makes Kerber different from recent failed rivals? Well, for starters, she has already accomplished task one: Beating Williams in a Grand Slam. Kerber won a three-set affair in the Australian Open final this year.

Second, she fights fire with fire in a way Williams has only seen from Venus. Kerber is listed at 5-8 and 150 pounds, nearly identical to Serena's 5-9 and 155 pounds. Although she lacks height, Kerber, like Williams, relies on fierce groundstrokes to wear down opponents. On top of that, she comes from the left side. Challenging the Williams sisters on hard court or grass has been a death wish for the better part of the past decade. But Kerber is a European constructed for fast play.

Angelique Kerber

In 2016, Kerber's confidence is through the roof. She has two titles this year, including the Australian Open, and she has reached four other tour finals. She fell to Williams in two sets at the Wimbledon final and she earned a silver medal in Rio, losing to Monica Puig in three sets in the gold medal match.

"I'm not thinking about quarters, semis or whatever," Kerber said after beating Vinci. "I'm just going there to play a good match and to win the match. I know that I can beat everybody, and this is what also gives me a lot of confidence and motivation for going out there and playing with a lot of emotion."

Kerber started the year ranked No. 10 in the world and jumped up to No. 2 after Wimbledon. If she wins the U.S. Open and Williams fails to make the final, Kerber would become the No. 1 player in the world. Williams has held the top spot since Feb. 18, 2013. No German woman has led the rankings since Steffi Graf on March 30, 1997.

Graf's 377 total weeks as No. 1 are the most all-time, although, at 309 weeks, Serena is closing in. Her 186 consecutive weeks right now are tied with Graf for the most in one stretch.

"This would mean a lot to me," Kerber says of being world No. 1. "I mean, when I was a kid I was always dreaming to be No. 1. Let's see. I mean, there are still matches to go. Serena has to play, as well."

Even if both make the final and Williams preserves her ranking, a Kerber title would inch her closer to snatching the crown from Serena before year's end.

Kerber is humble and charming off the court. She is talented but not arrogant. She knows she can be a dominant player. She knows she can beat Williams in the American's own building. But she is not about to start a war of words.

Five years ago, Kerber actually made her first Grand Slam semifinal in Flushing, charging through the draw as an unranked 23-year-old before losing to eventual champion Samantha Stosur.

"I had nothing to lose," Kerber remembers. "I came here. I just played great tennis. I think a lot of things [changed] since then. Now I'm a completely different player, I think. I have a lot of confidence. I know how to win big matches. I know how it feels playing on the stadium.

"I'm also enjoying it more than years ago. I'm trying to go there and enjoy the atmosphere and also win the matches. Years ago, I came here and I had nothing to lose and my goal was playing good, and now my goal is winning the matches."

Other players can say what they want, but only two players could legitimately say they arrived in Queens with the feeling of championship or bust: Williams and Kerber.

Kerber has a semifinal match looming against either two-time finalist Caroline Wozniacki or No. 48 Anastasija Sevastova. Meanwhile, Williams has a challenging quarterfinal matchup against No. 5 seed Simona Halep. After that, it would be No. 92 Ana Konjuh or No. 10 seed Karolina Pliskovain the semis. If Kerber and Williams do their job, the top two players will meet in the final Saturday.

And then we will really see what this rivalry is made of.