Sept. 4 (Bloomberg) -- Rafael Nadal advanced to the U.S. Open semifinals for the first time and Serena Williams beat older sister Venus to complete the women's final four at the last Grand Slam tennis tournament of the year.

Nadal, the men's top seed, lost the first set to American Mardy Fish before completing a 3-6, 6-1, 6-4, 6-2 victory in a match that ended at 2:10 a.m. at the National Tennis Center in New York.

The next opponent for the 22-year-old Spaniard is Britain's Andy Murray, who beat Argentina's Juan Martin del Potro in four sets to advance to a Grand Slam semifinal for the first time.

Murray and Del Potro played 3 hours, 58 minutes and the Williams sisters finished in just under 2 1/2 hours, pushing Nadal and Fish back to an 11:30 p.m. start in Arthur Ashe Stadium. The finishing time for the last match was 16 minutes shy of the tournament record of 2:26 a.m., set in a 1993 meeting between Mikael Pernfors and Mats Wilander.

``The night session here is always unbelievable, always amazing,'' Nadal said in a televised interview. ``I prefer to finish a little earlier, but I'm going to be fine.''

Serena Williams rounded out the women's semifinalists with a 7-6 (8-6), 7-6 (9-7) victory over her elder sister. With the win, she takes a 9-8 lead against Venus all-time and advances to meet Russia's Dinara Safina, who eased into the semifinals with a 6-2, 6-3 win over Flavia Pennetta.

``It definitely felt like a final,'' Serena said in a courtside interview. ``It was so hard.''

Today's Schedule

Play should finish earlier today at the tournament, where the last two men's semifinalists will be determined.

Four-time defending champion Roger Federer of Switzerland plays Gilles Muller of Luxembourg in the afternoon session and American Andy Roddick, the 2003 winner, meets Serbia's Novak Djokovic in the night match.

Nadal started slowly against Fish, losing his first service game. It was the first time in five matches that Fish had broken Nadal's serve.

The Spaniard rebounded in the second set to take control of the match and finish with 19 unforced errors to 53 for Fish. Nadal is the fifth Spaniard to reach the U.S. Open semifinals, joining Manuel Orantes (1975), Carlos Moya (1998), Juan Carlos Ferrero (2003) and David Ferrer (2007). Orantes was the only one to win the title.

Murray, 21, is the third British male to reach the final four after Greg Rusedski in 1997 and Tim Henman in 2004.

Sixth-seeded Murray, who is 0-5 all-time against Nadal, held on for a 7-6 (7-2), 7-6 (7-1), 4-6, 7-5 win yesterday over 19-year-old Del Potro, who was the youngest player remaining in the men's draw.

Tiebreakers

Murray dominated the first two tiebreakers and then broke serve for a 3-1 lead in the third set. Del Potro fought back with two service breaks to take the set and broke Murray again to start the fourth. Del Potro then lost his serve three times, including once to end the match. It was his first defeat in five tournaments -- a streak of 23 matches.

Murray, the fifth British male to reach a Grand Slam semifinal in the Open era, is guaranteed a career-high ranking of No. 4 after the tournament. It would match the highest ranking for a British man since computer standings were introduced in 1973. Rusedski and Henman reached as high as fourth in the world.

Aiming Higher

``Getting to the semis is a great achievement but I want to go a little bit further,'' Murray said.

The 1999 and 2002 U.S. Open champion, Serena Williams rallied in both tiebreakers to oust her older sister. She trailed 6-4 in the first-set tiebreak and overcame a 6-3 deficit in the second.

Serena, the 26-year-old No. 4 seed, saved eight set points in the second set before winning the match when a video review showed her sister, the No. 7 seed, sent a forehand just long.

No. 6 seed Safina needed a little over an hour to eliminate Italy's Pennetta in yesterday's first singles match. The French Open runner-up had 25 winners to Pennetta's 13.

The sister of 2000 U.S. Open men's champion Marat Safin, Safina's previous best at the tournament was a quarterfinal appearance in 2006.

``I'm getting closer to reaching the same thing as my brother,'' Safina said after the match. ``I hope one day we could have the same title.''

Safina joins another Russian, fifth-seeded Elena Dementieva, and Jelena Jankovic, the second seed from Serbia, in the semifinals. The women's semifinals are scheduled for tomorrow, with the men's semifinals on Saturday.

To contact the reporter on this story: Erik Matuszewski at the National Tennis Center in New York at matuszewski@bloomberg.net

Last Updated: September 4, 2008 02:40 EDT