Andy Murray roared in relief as he celebrated reaching his first grand slam semi-final after ending the U.S. Open campaign of Argentine teenager Juan Martin Del Potro 7-6, 7-6, 4-6, 7-5 on Wednesday.

The 21-year-old Murray, dominant in the tiebreakers, winning the first 7-2 and the second by 7-1, overcame shaky moments in an error-filled third set before ending the slugfest as Del Potro served to stay alive.

"I'm very relieved," Murray said after the grudge match against the in-form, 19-year-old Argentine, with whom he traded on court insults at the Rome Masters in May.

"I had my chances in the third set and let it slip a little bit. He had been on such a great run this summer. He fought so hard, and it was a great atmosphere in the end."

Murray snapped Del Potro's 23-match winning streak that included wins in his last four tournaments -- on clay at Stuttgart and Kitzbuhel, and on hard courts at Los Angeles and Washington that sent his ranking from 65 to number 17.

Russian sixth seed Dinara Safina also registered a personal first at Flushing Meadows when she overpowered Italian Flavia Pennetta 6-2 6-3 to reach the semi-finals of the U.S. Open for the first time.

Their triumphs on another hot New York day served as prelude to a glittering night programme at the National Tennis Centre.

The opening match under the lights was scheduled to feature the Williams sisters, fourth-seeded Serena and Wimbledon [Images] champion Venus, staging their 17th career showdown with the winner advancing to the last four against Safina.

Serena and Venus, both double Open champions and the only women's players left in the draw with any grand slam crowns, had split their previous 16 career meetings and were 5-5 against one another on the grand slam stage.

Murray's obstacle to a first grand slam final was to be determined in the final match of the day when top-seeded world number one Rafael Nadal [Images] took on unseeded American Mardy Fish [Images].

Sixth-seeded Murray lost to Nadal in straight sets in the Wimbledon quarters, the only previous time he had appeared at that stage of a slam.

After venting his frustrations by slamming his racket to the ground, Del Potro came alive when Murray moved within two games of victory in the third set.

Ripping a series of stinging passing shots he broke Murray for 5-4 and wrapped up the third set with a crisp forehand.

Both players appeared reluctant to snatch control of the match in the fourth until Murray pounced on a forehand error by Del Potro to earn two match points.

The first went begging but the Scot let out a roar when a Del Potro backhand floated wide on the second after nearly four hours of action.

With the victory, Murray was guaranteed to rise to fourth in the world, matching Greg Rusedski [Images] and Tim Henman for the highest-ever ranking by a British man since rankings were introduced in August 1973.

Murray and Del Potro had a warm exchange at the net, full of mutual respect after the hard-fought contest.

"I said congratulations and good luck for the next match," said Del Potro, who was bandaged above and below his left knee and was limping toward the end of the quarter-final.

"I just did my best. He played an unbelievable match. He's a great player and that's it."

The 22-year-old Safina, runner-up at the French Open and the Beijing [Images] Olympics [Images], raced through the first set and came from a break down in the second to clinch a place in the last four.

"It's great," Safina said in a courtside interview. "I'm getting closer to reaching the same thing as my brother (Marat Safin [Images], the 2000 champion), so I hope that one day we can have the same titles."

While Safina was longing for a brother-sister double of U.S. Open singles titles, Leander Paes [Images] was also seeing double at the National Tennis Center.

Nearly a decade after capturing two doubles titles at Wimbledon in 1999, 35-year-old Paes remained on pace to replicate the feat at Flushing Meadows.

Paes and his Czech partner Lukas Dlouhy sprinted into the men's doubles final by pulverising Argentine duo Maximo Gonzalez and Juan Monaco 6-2 6-0.

The success came less than 24 hours after Paes and Zimbabwean Cara Black advanced to the mixed doubles showpiece.

• Complete tennis coverage
• All you want to know about Sania Mirza