Flushing Meadows, NY -- (Sports Network) - Serena Williams took advantage of a inopportune Venus Williams, beating her older sister, 7-6 (8-6), 7-6 (9-7) to reach the semifinals at the U.S. Open.

Venus, who bested Serena in July's Wimbledon final, blew 10 set points in the match and had 45 unforced errors in the 2 hour, 25 minute encounter.

"I can't believe I won," said Serena. "It's difficult because she's such a great player. She was serving balls 125, 127 miles-per-hour and I was trying to stay with her."

Serena, the 1999 and 2002 U.S. Open champion, went ahead 9-8 in the all-time series with Venus, also a two-time titlist here.

The eight-time major champion Serena stole the first set. Not only was she behind 5-3 in games, but she won the final four points of the tiebreaker. Venus let one set point literally slip away when she lost her footing and a forehand return settled into the net. Later, Serena came through with a net winner and she finished out the set when her sister went a forehand return wide.

"When I got down I just relaxed," said Serena, who advanced to the semifinals at Flushing Meadows for the first time since 2002. "I started running all sorts of balls down."

Venus, a seven-time major titlist, was ahead 5-2 in games in the second set and was up 40-love trying to close it out at 5-3. The younger sister though wouldn't wilt and Serena came up with five straight points, taking advantage of wide and long returns from Venus.

"I tried not to look at her because if I look at her I might start feeling sorry," said Serena. "I caught myself looking at her when I broke back in the second and I said, 'Serena keep your head down.' I want the best for her. I love her so much because she's my best friend."

The tiebreak turned in Venus' favor at 6-3, but she continued to blow set point chances, most notably going wide on an overhand slam try. That squared the tiebreak after 12 points. Serena lost the following point, but came back with a net winner and after a long return by her sister, she closed out the match when Venus sent a forehand long.

"I felt like I was always in control. I was always playing more aggressive, always going for a lot more, but just always coming up a little short," said Venus.

Next up for the 26-year-old Serena will be surging Russian Dinara Safina, who was an easy quarterfinal winner Wednesday. The sixth-seeded Safina subdued 16th-seeded Italian Flavia Pennetta 6-2, 6-3 in 71 minutes on Day 10 at the USTA Billie Jean King National Tennis Center.

The 22-year-old Safina was sharp on Wednesday, reaching her first-ever U.S. Open semifinal by breaking Pennetta's serve on four occasions, compared to only one break for her Italian counterpart.

"I think today maybe I can say finally I'm happy with myself, the way I played, because really I did what I had to do," said Safina. "I was aggressive on the court. I was following the balls every time. I think today, finally I played my game."

The overmatched 26-year-old Pennetta was playing in her first-ever Grand Slam quarterfinal.

The high-flying Safina is seeking a trip into her seventh final in her last eight tournaments. She's already won three titles this year and was the runner-up at the French Open and Olympic Games in Beijing.

Safina has won 37 of her last 41 matches and beaten three different world No. 1 players during that span. The U.S. Open Series winner would collect $2.5 million with a title here in New York.

The other semifinal here will pit second-seeded Serb Jelena Jankovic against fifth-seeded Russian Elena Dementieva, the reigning Olympic gold medalist and 2004 U.S. Open runner-up.