Venus Williams glanced up at the giant screen, hoping for a change. What she wanted was for an overrule by an inanimate object, a machine that had no idea how hard she was fighting, how badly she wanted to win.

On the other side of the net, kid sister Serena was holding her index finger up, trying to indicate that Venus's final forehand was long. But it wasn't until the screen showed the white computerized ball slowly sailing just over the baseline that Venus walked to the net, smiled politely, shook hands, and then shoved her racket into its bag.

She was left to wonder how she had just squandered 10 set points, eight in the second set, and lost, 7-6 (8-6) 7-6 (9-7) in 2 hours 25 minutes. In a nightcap that lasted until after 2am, top seed Rafael Nadal dropped the first set, but went on to beat American Mardy Fish 3-6 6-1 6-4 6-2.

"This definitely felt like the final of the US Open," Serena said courtside, after Venus had headed to the locker room. "It was so hard. I can't believe I won."

There would be no hug at the net this time. The disappointment was simply too great for Venus, who broke serve early in the first set, went up, 5-3, but then lost her serve to even the set at 5-5.

When they went to a tie-breaker, Venus broke again and went ahead 6-4 on her serve. But two ground stroke errors by Venus brought Serena back to life, and she took full advantage by hitting an ace and a backhand volley winner. The set ended when Venus hit a forehand down the line that went wide.

It was a match that began with little emotion and drama, largely because the sisters appeared lifeless and listless. During the warm-up, Venus lobbed sitters to her sister at the net and Serena smacked the first four long, including one that ticked off her racket and hit her on the head. The air around the stadium felt heavy and tense, and the sellout crowd of 23,763 had trouble choosing a favorite, with one vocal woman cheering for Venus one minute and Serena the next.

All of that changed midway through the second set when the quality of play by both sisters tangibly lifted. By then, Venus had outmuscled and outvolleyed Serena to a 5-3 lead and a third set seemed imminent. But Venus failed to capitalise on two more set points, and yet another one in the 12th game, which featured five deuces as well. Serena forced the tie-breaker on a wild forehand by Venus.

"I've never had a match like this in my life," said Venus, who reached 6-3 in the tie-breaker but couldn't hold on, not even when Serena lofted an eminently getable topspin lob at 5-6, only to see Venus pound the overhead wide. "I feel like I'm pretty good at handling emotions. I'm a very good closer, so today was... I guess there's always a first."

Serena took advantage of her first match point when Venus pushed her forehand just long. By then, the audience was on its feet, wildly appreciative of the high level of this, the 17th career meeting between the sisters as professionals, the one that gives Serena a 9-8 edge. The sisters have faced each other 11 times in Majors, including four times at the US Open, and each has won twice.

While some felt this was, in many ways, the final of the tournament, others would disagree, especially those women left in the draw. To reach the final, fourth-seeded Serena must now take on sixth-seeded Russian Dinara Safina, who needed just 1 hour 11 minutes to dismantle 16th-seeded Flavia Pennetta of Italy 6-2 6-3 in the day's earlier quarter-final.

Safina, the most dominant player on the WTA Tour this summer, has reached the final of six of her last seven events, including the French Open, where she lost to Ana Ivanovic, and the Olympics, where she fell to fellow Russian Elena Dementieva. Dementieva will play second-seeded Jelena Jankovic in the other semi-final.

"Today, I can say I'm finally happy with myself, the way I played, because I really did what I had to do," said the 22-year-old Safina, who, as winner of the US Open Series, stands to earn $2.5million should she capture the title here. "I was aggressive on the court. I was following the balls every time. I think it was today, finally, I played my game."

Safina and Serena have met on four occasions, not including a quarter-final in Paris in 2005 that Safina won in a walkover. They also played at the French Open last year and here in 2002, with both matches won by Serena. In 2002, Serena went on to win the US Open, the last time she captured the title here. Safina won on clay in Berlin earlier this year.

Regardless of the outcome of the Williamses' match, the sisters have redefined the sport. They were a focal point in the WTA Tour's new Roadmap 2009, a revamped and streamlined calendar that was revealed yesterday by Tour CEO Larry Scott.

Designed to shorten the season by two weeks and create a much-needed offseason in November and December to allow players to heal their too-oft-injured bodies, the new schedule will feature 54 tournaments in 31 countries with record prize money of more than $85million.

In addition, nearly half of the tournaments will be combined events with the men's ATP Tour or be held back to back with the men. Ten of those will feature equal prize money, including five - in Indian Wells, California, Miami, Madrid, Beijing, and the season-ending championships in Doha, Qatar - that require mandatory participation or a player can risk suspension.

The plan, which requires a top player to commit to 10 premier events rather than the current 13, is designed to ensure that more of the marquee players compete against each other at the same events. It will also allow the players to share in revenue generated by those tournaments and will launch a new ranking system that rewards players for competing in the most important events and penalises them for their failure to compete, regardless of whether they are injured or not.

For the Williams sisters, that presents a conundrum because of their refusal to play the Pacific Life Open in Indian Wells since 2001, when they accused fans of directing racial epithets their way following Venus's last-minute default to Serena in the semi-finals. Under the new rules, one way for the sisters to avoid suspension for not playing would be for them to make promotional appearances on behalf of the very tournament they refuse to participate in.

"I'm hopeful they will play [Indian Wells], but at this point in time I don't think they'll play based on what they told me," said Scott. "But they will have to participate in these activities to avoid suspension. I'm confident we'll figure out a way to use their in-market appearance in a way that's good for the sport and not negative for them or negative for the tournament."