CINCINNATI, OH, USA - Although the Williams sisters are still going strong and Bethanie Mattek-Sands only seems to be getting better, the questions have been flowing over the last few years about the future of American tennis - that future may be 19-year-old Christina McHale, who stunned the No.1 player in the world on Wednesday, beating Caroline Wozniacki, 64 75.

"You guys really helped me," McHale told the crowd in her on-court interview. "I still can't believe it right now. I'm trying to soak it all in, I'm so excited."

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McHale, who is the youngest player in the Top 100 - she is actually the only 1992 baby in the Top 100, everyone else 1991 or earlier - was at her grinding best against an error-prone Caroline Wozniacki, using a mix of flat and loopy rally shots to prolong the points from the baseline, forcing numerous errors.

Serving for the match at 64 65, McHale drew four errors from the Dane - first a long backhand, then a wide forehand, then a backhand return into the net, and finally a big forehand mis-hit that sailed well past the baseline. McHale had never even played a Top 4 player before, let alone defeat a Top 10 player (although she did have a retirement win over Victoria Azarenka in 2010).

"I was definitely excited to play her, and I knew the crowd would help me a little bit, being here in the States," McHale said. "I was just trying to take each point one at a time and not get too focused on who I was playing or the moment."

McHale is currently ranked No.76 in the world, the No.4 American after No.31 Serena Williams, No.33 Mattek-Sands and No.36 Venus Williams.

Wozniacki was looking for her 50th match win of the year, and would have crossed the $10 million career prize money mark too. Instead she lost her opening match for the second straight week, having fallen to Roberta Vinci after a first round bye last week too, over in Toronto. This is the first time since the 2009 season Wozniacki has lost back-to-back opening matches.

"I think I made 37 unforced errors; normally I'm seven or eight in a match," Wozniacki said. "It happens. You have good patches and some rough patches. I'm going to come back and win a lot of matches in the future. Of course I'm disappointed and I would have liked to win, but it's only this week. Life goes on."

Speaking of Serena Williams, the 13-time Grand Slam champion, who has gone 12-0 during the summer hardcourt season, had to withdraw prior to her second round match against No.10 seed Samantha Stosur due to a right toe aggravation. "It's not nearly extreme, it's just a bit of an aggravation because I think I've been pounding it a lot and playing a lot," Williams said. "I don't think this is a good time for me to take a big chance. I just don't think that's smart.

"I'm definitely disappointed, but at the same time I have a lot to look forward to."

Williams was asked if this would affect her chances of playing the US Open. "If anything, it'll make them better, maybe because I have more opportunity to rest up and get 200% healthy, which could be a very dangerous thing."