"Down that first set, it's comforting to know if anything goes wrong, Serena Williams is going to play the second match," Venus said.

Venus rallied to win, and Serena completed a sibling sweep today, leaving the U.S. Fed Cup team on the verge of beating Belgium.

Playing Fed Cup for the first time since 2003, Serena beat Caroline Maes 6-1, 6-4 to give the Americans a 2-0 lead in the best-of-five opening-round match.

Venus struggled early with gusty conditions and fell behind 5-3 before winning 11 consecutive points, then beat Kirsten Flipkens 7-5, 6-2.

"With Venus on our team, we're guaranteed two wins," Serena said. "So if I can just win one match, we pretty much have it in our pockets."

Singles matchups will be reversed Sunday, followed by doubles, and Serena can clinch a U.S. victory by beating Flipkens. The winning team will face the Russia-Spain winner in the semifinals following Wimbledon in July.

Russia led Spain 2-0 in Moscow. France and Italy, both playing at home, also took 2-0 leads.

The Americans were heavily favored due to the absence of Belgium's top players, top-ranked Justine Henin and No. 4 Kim Clijsters, who declined to take part. The Williams sisters often skip Fed Cup themselves, but they agreed to play this weekend in part because Delray Beach is only a half-hour drive from their home in Palm Beach Gardens.

Friends, family and designers from Venus' fashion business were part of the crowd of 3,415, and the sisters gave them plenty to cheer about.

"It was really windy, and balls are bouncing funny," Serena said. "But America is up 2-0, and we hope to improve it Sunday."

Serena took a 5-0 lead after only 17 minutes, and when Maes held for 5-1, the Belgian grinned at the achievement. Serena never faced a break point, slammed backhand winners into both corners and moved well, showing no signs of the groin injury that forced her to retire from a match last week at Charleston, S.C.

She wore tape on her right thigh as a precaution.

"I'm really excited that I didn't have any pain in the groin," Serena said. "I was really nervous I would."

She won points even when Maes had her on the run, backpedaling after a lob and smacking an overhead winner from near the baseline, then yanking a running backhand lob crosscourt for another winner.

Staging a career comeback after a series of long layoffs, Serena improved to 16-2 in 2007. She has won the year's two biggest tournaments, the Australian Open and Key Biscayne.

"She's the best player in the world when she really wants to," Maes said.

Venus flirted with trouble in the opening match. Starting slowly, she was three points from losing the first set.

Flipkens wavered, double-faulting to lose serve and make it 5-4, then double-faulting again on break point to fall behind 6-5. Venus closed the set with an overhead slam, then pulled away in the second set, winning the final five games.

Venus said she struggled early with the wind and hurried some shots.

"I was rushing because I wanted to hit great shots," Venus said. "Sometimes it's not about the great shots. Sometimes it's about the good shot. I love greatness, but sometimes it's goodness."

Flipkens, ranked No. 132, looked relaxed at the outset, started with an ace and won the first game at love. Conditions were so windy that the hem of Venus' skirt occasionally blew up around her waist, and she double-faulted three times to fall behind 4-2.

But Venus began to hit her groundstrokes more confidently as the match progressed, and she won her final six service games.

"She was on fire in the second set," Flipkens said. "I think she had problems in the first set with the conditions. She was serving way better in the second set and proving she's one of the top players in the world."

At Moscow, No. 5 Svetlana Kuznetsova No. 9 Nadia Petrova won for Russia in straight sets. They're the highest-ranked players competing this weekend among the quarterfinalists.

At Limoges, France, Tatiana Golovin and Nathalie Dechy also won in straight sets. The team advancing will take on Italy or China in the semifinals.