Federer defeated the 28th seeded Nicolas Kiefer of Germany 6-4, 6-1 while Williams beat the 32rd seeded Shuai Peng of China 7-5, 6-2. Williams
moved to a 49-5 career Miami record.
Last year Federer lost to the American Andy Roddick in the quarterfinals 7-6(7-4), 4-6, 6-3. The Swiss took the Miami singles title in 2005 and 2006 after first topping the ATP Tour rankings in February of 2004.
Federer held the number one ranking for a record 237 consecutive weeks before the current world number one, Rafael Nadal of Spain unseated him in August of last year. Federer in recent months struggled to defeat Nadal; losing the the Spaniard in the finals of the French Open, Wimbledon and the Australian Open.
Safina is the younger sister of Marat Safin. Safin reached the world's top ranking in November of 2000. Safina first had the opportunity to reach the top ranking weeks ago in Indian Wells, should she had reached the tournament final.
The Russian not played her best tennis over the years in Miami as she had never advanced past the round
of sixteen before her last year's quarterfinal finish.
Safina reached the French Open final last year before losing to Williams in January in the final of the Australian Open. She'd also won 15 matches in a row before losing to her fellow Russian Elena Dementieva in the Summer Olympics final.
Stosur, ranked 42nd in the world, called the win which came a day before her 25th birthday the most important of her career.
“I've been trying to get a win like this for a long time. I was happy I was able to do it and hopefully there will be more to come,” Stosur said. “When I started playing well again I thought it was possible. Now I'm going to really enjoy it.”
Safina could have claimed the WTA Tour's number one ranking had she won the Miami tournament.
“Of course it's very disappointing,” Safina said. “That's what I'm playing for. I want to be there. Suddenly I go to the court and I don't even fight for it. It's just bad behavior from my side. The champions, they don't do this. It's a good punishment for me. I have to get smarter.”
With Safina's loss, the winner of the Serena Williams versus Venus Williams semifinal now is a heavy favorite to win the women's tournament.
Federer could face Roddick in the quarterfinals before a possible matchup versus Nadal in the men's final.