The last time VENUS WILLIAMS had a chance to stop SERENA WILLIAMS from completing a GRAND SLAM? That would be two months ago, when the sisters faced each other in the round of 16 at WIMBLEDON, with a potential "SERENA SLAM" at stake. Today a potential straight-up, calendar-year, this-is-the-real-thing GRAND SLAM will be in the air as VENUS tries to halt SERENA's historic march in the U.S. open quarterfinals. I've lost sleep trying to figure out who's under more pressure, the WILLIAMS who needs this win and two more to complete her dream year, or the WILLIAMS who just needs this one to end her sister's dream year. And based on the amazing poise both have shown in the past few days, I seem to have spent more time thinking about this than either one of them has. So the real question, I guess, is who is DAD rooting for?... The experts' take on the matchup... What if one of them, or anyone in the U.S. OPEN, suddenly hears nature's call in the middle of a match? Turns out I'm not the only one who stays up at night worrying about THAT... SERENA has won 15 of the sisters' 26 head-to-head matches... Is there any other major sport that schedules potentially three- or four-hour matches to start at 9 pm? Actual start time of