Three years ago he was a gawky teenager who suffered cramp towards the end of his longer matches. As Andy Murray prepares to face Stanislas Wawrinka in tonight's showcase match at the US Open, the Scot is justifiably proud of his new-found reputation as one of the game's never-say-die strongmen.
Two months after rolling up his sleeve and flexing his biceps to celebrate coming back from two sets down to beat Richard Gasquet at Wimbledon, Murray made the same gesture to his coaching team at the end of another thrilling fightback here on Saturday night. The world No 6 was two points from defeat against Austria's Jurgen Melzer but rallied to win 6-7, 4-6, 7-6, 6-1, 6-3 after three hours and 52 minutes.
Tim Henman was 29 when he first won from two sets down, against Cyril Saulnier at the 2004 French Open. His 21-year-old successor as Britain's flag-bearer has now done so three times: against Israel's Andy Ram in the Davis Cup at Eastbourne two summers ago, against Gasquet at Wimbledon, and now here. He has won seven of his last eight five-set matches, suffering his only defeat against Rafael Nadal inlast year's Australian Open.
Murray considered his latest back-from-the-dead feat a greater achievement than his win over Gasquet. "Melzer was playing so well that it was going to be really tough for me to get back into the match," he said. "Gasquet was obviously playing great, too, but I still felt like I had a shot at getting back in there. I knew that in the past he had struggled to close big matches out and struggled over five sets."
Melzer, who will face Murray again at Wimbledon in the Davis Cup in three weeks' time, went for broke and hit some outstanding shots in the first three sets, but there was a lack of sparkle in Murray's early play. The Scot's return of serve is usually one of his strongest weapons, but he struggled to find his rhythm. Melzer also played better in the big moments, saving the first nine break points against him.
Murray's response was magnificent. At 5-5 in the third set tie-break he hit the biggest serve of his life, a 138mph ace, and followed it with a backhand winner down the line. Thereafter he took command. While the world No 48 was receiving treatment for cramp throughout the final set, a pumped-up Murray looked stronger than ever.
The past criticism of his fitness used to rankle with Murray. "When you're 18 years old, a teenager, you're not going to be at your physical peak," he said in the wake of Saturday's victory. "It all came upon me very quickly playing in the Slams. I went from playing Futures level tennis to the main tour within a week or two. It's not just physical but the mental aspect, because you're more nervous and pumped up as well. That can tire you out too.
"Everyone takes time to get stronger. If you asked Federer he'd say he's much better than when he was 18. Nadal is the exception, but even he tired in some matches when he was younger. Everyone does when they're young. You've got to let your body mature. If you do too much too early you're going to have injuries later. Now that I've finished growing I can do more weights and train harder. When you're 17 and 18, I think it's tough on the body to push so hard."
It has long been an ambition of Murray's to play in the cavernous Arthur Ashe Stadium and he will realise it tonight. His fourth-round meeting with Wawrinka is the second match of the evening session, which starts at midnight BST. It will give both players more time to recover from weekend efforts, Wawrinka having also come from two sets down to beat Italy's Flavio Cipolla, the world No 142.
"I have to improve as I haven't played that well in my last three matches," Wawrinka said as he looked forward to meeting Murray. "There will certainly be lots of ups and downs. There usually are in our matches. I know he's a fighter, that's for sure."
The two have become good friends and often practise together. They have already played each other four times this year, Murray winning all three of their matches on hard courts and Wawrinka, the world No 10, coming out on top on clay.
In the quarter-finals the winner will play either the Japanese, Kei Nishikori, who knocked out David Ferrer, the No 4 seed, or Argentina's Juan Martin del Potro, who has won his last four tournaments and is unbeaten in 22 matches. Rafael Nadal, who has yet to drop a set, should await in the semi-finals.
Roger Federer reached the last 16 in the other half of the draw with his best performance yet. The defending champion beat Radek Stepanek, the No 28 seed, 6-3, 6-3, 6-2, avenging his defeat by the Czech in Rome four months ago.
Federer is on course to meet Nikolay Davydenko in the quarter-finals. Davydenko beat his fellow Russian Dmitry Tursunov and now plays Gilles Muller, who became the first player from Luxembourg to reach the fourth round of a Grand Slam event when he beat Spain's Nicolas Almagro. Andy Roddick continued his progress with a straight-sets win over Andreas Seppi.
In the women's competition the highest seed left is Jelena Jankovic, who became the first to reach the quarter-finals when she beat Denmark's Caroline Wozniacki 3-6, 6-2, 6-1. The world No 2 now plays Austria's Sybille Bammer, who beat Marion Bartoli.
l Jamie Murray and his partner, Liezel Huber, are through to the mixed doubles quarter-finals after beating Kveta Peschke and Pavel Vizner 7-5, 6-3.