When Serena Williams began her Grand Slam march at the Australian Open in January, she had to beat American Madison Keys in a highly competitive semifinal match. Williams had this to say after the victory:

"It was an honor for me to play someone who will be No. 1 in the future."

Lofty words for Keys, 20, who will get a second chance on a big stage against Williams Sunday in the fourth round of the U.S. Open. Williams again will have to beat America's next best hope in women's tennis as she tries to complete the calendar year Grand Slam.

InfographicSerena and the SlamsU.S. OpenU.S. Open: Women's results

Williams is by miles the top-ranked player in the world. Keys is ranked 19th, and that makes her the second-ranked American. She is having her best Grand Slam season, reaching the semifinals at the Australian, the third round at the French and the quarterfinals at Wimbledon. She's on her own roll to the top even if she has miles to go to taste the Grand Slam accomplishments of Williams.

The match in Australia, which Keys lost, 7-6 (5), 6-2, gave her both an introduction to the most powerful women's player of all time and that kick of confidence that she can stay on the court with Williams.

"It was a great match. It was a really close first set," said Keys, who beat Agnieszka Radwanska in the third round. "The thing I remember most is saving a lot of match points."

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The native of Rock Island, Illinois, is under the tutelage of Lindsay Davenport, a three-time Grand Slam winner, including the 1998 U.S. Open. Davenport played Williams 14 times, winning four matches. She lost to Williams in the semifinals of the 1999 Open, in which Williams eventually won her first Grand Slam title.

"Anything that Lindsay has said from past experience may be beneficial," Keys said. "But I think Serena's game has evolved so much. She's one of the greatest tennis players of all time. You can sit there and you can watch her and watch every single match she's ever played . . . [and say] that's a big weakness. I'm going to play it there. I think if that was possible, she wouldn't be going for her calendar-year Slam."

By the time Williams was 20, she had won four Grand Slam events, including the Australian, French and Wimbledon in 2002. Keys' results are lagging by comparison, with only one WTA Tour event title. That said, she doesn't seem lacking in promise or confidence and sees her game as similar to Williams'.

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"I think we both have our serve as our weapon, and definitely our forehand," Keys said. "I think we are both pretty aggressive. She has the advantage of winning lots of Grand Slams and tournaments."

In recalling her match against Keys in Australia, Williams said, "I really think she played a high- intensity match. I thought she served well . . . She does everything really well. She has a good return. She has a heavy shot. She's moving well."

Of the spectrum of Williams' considerable talents, the one Keys finds both admirable and crucial is her undeniable competitiveness.

"I think just her determination is unlike anyone else's," Keys said. "You could be watching a match and she'd be down 6-0, 5-0, 40-love, and you still think she's going to win . . . I think her fight is something I kind of want to emulate and try to get as good as."

It will help that Keys already has played one match on Ashe, her Wednesday victory over Tereza Smitkova. It's a court that takes experience to play, both for the playing conditions and the nature of playing the game's best players.

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"It was really good that I was able to get out there before getting thrown out there with Serena, who has had so many matches out there," Keys said. "So being a little bit more comfortable with it is big for me."

As for what she needs to do to derail Williams' Grand Slam express, she said, "I'll let you know if I figure it out in a couple of days."