Defending champion Serena Williams survived against Victoria Azarenka in a classic comeback win today, preventing back-to-back upsets after her sister Venus lost to Li Na.

Venus Williams had a chance to serve for the match in the second set against Li but she went down in a flurry of errors.

Serena Williams lost the first set and was down two breaks of serve against Azarenka until she rallied to win five straight games to get herself back on track for a 4-6, 7-6 (4), 6-2 win.

"I'm surprised. I didn't expect to win when I was down 0-4. I was like, 'Oh well, at least I'm still in doubles,' " said Williams, who admitted she'd even started thinking about the trip home.

"That's not what you're supposed to think but that's what I was thinking."

Now, instead of the projected sisters semifinal on the top half of the draw, it will be: Serena Williams vs. No. 16 Li of China. Serena's close finish was only 18 hours after a knee problem forced men's defending champion Rafael Nadal to quit his quarterfinal against Andy Murray while he trailed in the third set.

Li rallied from a set and a break down, breaking Venus Williams' serve to stay in the match in the second set, on the way to her 2-6, 7-6 (4), 7-5 triumph.

"Best day of my whole life," Li said, grinning.

Also today, top-seeded Roger Federer rallied from a first-set loss to defeat sixth-seeded Nikolay Davydenko, 2-6, 6-3, 6-0, 7-5, to advance to the semifinals. Federer will play the winner of the match between third-seeded Novak Djokovic and 10th-seeded Jo-Wilfried Tsonga.

BASEBALL

A'S SIGN SHEETS

The Oakland Athletics added another veteran arm to their young rotation, agreeing to terms yesterday with free-agent righty Ben Sheets on a $10 million, one-year contract.

The 31-year-old Sheets has been plagued by injuries. The four-time NL All-Star didn't pitch in the majors last season while recovering from elbow surgery but last week held a pitching session for several scouts in Louisiana.

A four-time All-Star, Sheets becomes the team's second-highest paid player behind six-time Gold Glove third baseman Eric Chavez.

FOOTBALL

BOWL REPLACEMENTS

San Francisco 49ers defensive tackle Justin Smith has been added to the NFL Pro Bowl roster in place of injured Kevin Williams of Minnesota.

Atlanta got a representative for Sunday's game in Miami when receiver Roddy White was selected to replace Minnesota's Sidney Rice.

AND WHAT'S MORE ...

Six years after signing Miguel Tejada to a top-dollar contract, the Baltimore Orioles welcomed back the 2002 AL MVP with a $6 million, one-year contract yesterday after passing a physical. ... Texas Rangers closer Frank Francisco reached an agreement yesterday on a one-year contract worth $3,265,000 to avoid an arbitration hearing.