Murray insists on doing the talking against Del Potro

When they last met, Andy Murray told Juan Martin del Potro to "watch his mouth". When they face each other here today at the US Open there will be no love lost between the 21-year-old Scot and the 19-year-old Argentine.

It was in Rome four months ago that the two men met for the only time as seniors. Del Potro retired with a back injury early in the final set, but their verbal volleys were more memorable than anything either of them did with a racket.

Murray was unhappy that his opponent had not apologised for hitting a ball straight at him and was further angered when Del Potro made comments about his mother, Judy, who was watching in the stand. Having been warned for swearing, Murray suggested in vain to the umpire that he might like to discipline the Argentine for speaking out of turn.

Nevertheless, as Murray looked forward in the small hours of yesterday morning to his first quarter-final at his favourite Grand Slam tournament, he insisted he would have no sense of wanting to complete unfinished business. "When you get on the court you've got to put your emotions aside and get the job done," Murray said. "Whether I like Del Potro or not really doesn't make any difference. When you get on the tennis court it's another match and you've got to win."

The two players have not talked since Rome, but Murray said he was unconcerned that Del Potro had not apologised. "I've known him since we were really young," he said. "I wasn't great friends with him before. I don't need to be friends with him now."

Of more concern will be Del Potro's remarkable winning run this summer. As the Scot was starting his fourth-round match against Stanislas Wawrinka here on Monday night, Del Potro was completing his 23rd win in a row, beating Japan's Kei Nishikori 6-3, 6-4, 6-3. The Argentine has won his last four tournaments and is unbeaten since Wimbledon.

It will be Murray, nevertheless, who will start as the favourite after his superb 6-1, 6-3, 6-3 victory over Wawrinka. The Scot has beaten better players than the world No 10, but in terms of the consistent level of his performance he rated it his best victory at a Grand Slam tournament.

Murray hardly put a foot wrong. Where he had started too cautiously in the previous round against Jurgen Melzer, Murray found the perfect balance here between defence and attack. Wawrinka never had time to settle as the Scot peppered the Swiss No 2 with aggressive returns of serve and pounding groundstrokes, with his backhand down the line a particularly devastating weapon.

The British No 1 said it had felt "pretty special" to play as well as he had in the 23,000-capacity Arthur Ashe Stadium, the biggest arena in tennis. As a teenager he had watched in awe when he attended his first night match in the stadium. It had been a long-term ambition to return as the headline act.

The victory ensured that Murray would climb next week to his highest-ever world ranking at No 5. He could yet rise to No 4, which was the highest position that Tim Henman and Greg Rusedski ever reached, and with more than two months of the season left he looks certain to have enough points in the bag to qualify for the season-ending Tennis Masters Cup.

Reaching the last eight equals Murray's best performance at a Grand Slam tournament following his run to the quarter-finals at Wimbledon two months ago, when he was beaten by Rafael Nadal. The new world No 1 would probably be his semi-final opponent if he beats Del Potro, though Nadal first has to beat the American Mardy Fish.

Murray, nevertheless, will take nothing for granted against the 6ft 6in tall Del Potro, a baseline player with big groundstrokes. "He doesn't miss a whole lot," Murray said. "He moves pretty well for a big guy. He does everything well. He's got a good cross-court backhand and just hits the ball hard and solid. He doesn't make a whole lot of mistakes."

Novak Djokovic reached the quarter-finals yesterday but only after a gruelling struggle against Tommy Robredo. The world No 3 defied stomach, hip and ankle problems to win 4-6, 6-2, 6-3, 5-7, 6-3 after nearly four hours. Gilles Muller, the world No 130, continued his remarkable run by knocking out Nikolay Davydenko, the world No 4, in five sets.

The first woman through to the semi-finals was Elena Dementieva, the Olympic champion, who brushed aside Patty Schnyder for the loss of only five games. In the other half of the draw the Williams sisters meet today in the quarter-finals, the winner to play Dinara Safina or Flavia Pennetta.

Murray's brother, Jamie, and his partner, Liezel Huber, reached the semi-finals of the mixed doubles with an excellent victory over the No 2 seeds, Katarina Srebotnik and Nenad Zimonjic. Last night they were playing the Americans Jill Craybas and Eric Butorac, who was Jamie's doubles partner until their split last year.

Del Potro's winning run

Stuttgart (clay)

Followed a disappointing Wimbledon by beating Richard Gasquet, the world No 15, to claim his first senior title. Philipp Kohlschreiber, world No 35, was his next highest-ranked opponent.

Kitzbuhel (clay)

Did not drop a set in five matches, though none of his opponents were ranked in the world's top 50. Beat Austria's Jurgen Melzer 6-2, 6-1 in a final that lasted less than an hour.

Los Angeles (hard)

Dropped only three games against Mardy Fish before final victory over Andy Roddick, although the world No 9 - his only top 10 opponent during his winning run - was recovering from a shoulder injury.

Washington (hard)

Beat Serbia's Victor Troicki in straight sets in the final. Tommy Haas, world No 48, was his highest-ranked victim in the tournament, while Israel's Dudi Sela was the only player to take a set off him.

US Open (hard)

Has reached his first Grand Slam quarter-final with victories over Guillermo Canas (world No 53), Thomaz Bellucci (86), Gilles Simon (16) and Kei Nishikori (126). Needed five sets to beat Simon, four against Canas and Bellucci, three against Nishikori.