Having spent much of the first half of this year insisting that a slowing-down in his progress was of no great concern, Andy Murray may now have to urge caution as he cuts a swathe through the game's leading players.
The Scot, 19, was due to face Andy Roddick in the quarter-finals of the Western and Southern Financial Group Masters in Cincinnati last night, having followed up his remarkable victory over Roger Federer by overcoming exhaustion and intense heat to beat Robby Ginepri on Thursday.
Whatever his final pay cheque from Cincinnati, Murray will leave with another substantial boost to his ranking. The nine Masters Series tournaments - Cincinnati is the year's seventh - carry many rankings points. A semi-final appearance in last week's Toronto Masters took Murray up 10 places to No 21 and the world's highest ranked teenager will be in the top 20 next week.
The emphasis the leading players put on the Masters Series was evident in the Cincinnati quarter-finals line-up. Juan Carlos Ferrero, the world No 31, was the only player with a lower ranking than Murray, the others being Rafael Nadal (No 2), Ivan Ljubicic (3), Tommy Robredo (7), Marcos Baghdatis (10), Roddick (12), Fernando Gonzalez (13) and David Ferrer (16).
Yet Murray's current run is in marked contrast to earlier this year. Having climbed to No 41 in the world in March after his American indoor campaign, the Scot's ranking remained in the mid-40s as illness and injuries hampered his progress through the spring. After a succession of defeats to journeymen like Jean-René Lisnard and Filippo Volandri, Murray's year reached a low with his 10th defeat in 13 matches against Janko Tipsarevic at the Queen's Club. However, he has since won 20 out of 25 (not including last night), reaching the quarter-finals at Nottingham, the fourth round at Wimbledon, the semi-finals at Newport, the final at Washington and the semi-finals at Toronto. Brad Gilbert, his new coach, started working with him in Washington.
Federer pinpointed the problem for players of Murray's age after their match on Wednesday. "Youngsters play good on the day," he said. "If they play good for the entire week, that's the other question."
The world No 1 became the youngest player in the world's top 100 at 18 and was long tipped as a future champion, but it was another three years before he started winning tournaments regularly. His senior progress was not dissimilar to Murray's: at 18 he finished the year as world No 64 (Murray was 65) and he finished the following campaign at 29 (Murray is 21). Federer won his first Grand Slam title two months before his 22nd birthday.
Although Nadal won the French Open two days past his 19th birthday, he is physically more advanced than his contemporaries. Novak Djokovic, Gaël Monfils and Richard Gasquet, who with Murray and Nadal form an exciting new generation, are all in the top 30 but have made similarly chequered progress and have all had physical problems.
The past three weeks have opened Murray's eyes to the demands on the men's tour. Even Federer, who won in Toronto, said that it was impossible to win Masters Series tournaments in successive weeks. Indeed, Murray and Gonzalez were the only quarter-finalists from Toronto who made it to the same stage in Cincinnati.
Against Ginepri, the world No 18, Murray took the first set on a tie-break after saving two set points. At one set all and 4-2 down, and with the temperature at 40C, Murray looked a spent force, but he showed great resilience and spirit to win 7-6, 2-6, 6-4.
"I felt a little bit ill after the first few games," the Scot said. "It shows that physically there is still a lot of work to be done, but I showed good heart to come through. But I shouldn't be letting myself get tired after three or four games.
"That's the most tired I've been on a tennis court. That was my 13th match in 16 days and I am just not used to it. My legs weren't there today. They were burning after each point. At the end I wasn't thinking what I was doing. I was just trying to get the ball back into court and chase every ball down."