So now we know. Roger Federer’s four-year - or 235-week - reign as world No 1 is almost over. The Swiss’s 7-6, 4-6, 7-6 defeat by Ivo Karlovic, of Croatia, in the third round of the Western & Southern Financial Group Masters, last night means that if Rafael Nadal wins the title in Cincinnati this week, he will usurp the Swiss to become the 24th man to top the ATP world rankings.

Having lost his Wimbledon crown to the Spaniard last month, Federer is now on the verge of losing the other thing most dear to him. And if it does not happen this week, then, depending on how far Nadal goes, it will almost certainly happen next week. Only 300 points separated the two at the start of this week and the 350 points that Federer won in reaching the final in Toronto last year drop off the computer on August 11, compared with 225 for Nadal.

At the very latest, August 18 – the beginning of the week when the seedings for the US Open are announced – will be D-Day.

For many, Nadal became the unofficial world No 1 the moment he denied Federer a sixth straight Wimbledon title. When it does happen, no one will be able to say he does not merit it.

“I’m looking forward to the next two tournaments,” a defiant Federer said. “Those are really the ones that can make this season from a good one to a great one again. I hope I can manage to pull off something in the next couple of weeks.”

Last week Federer had said: “If Nadal gets it [the No 1 ranking] he deserves it. He’s been No 2 for a long, long time. He had his chances in the past and now he’s closer than ever just because he could really get Wimbledon. That was a big one for him and he’s been on an incredible roll.”

When, come December, we reflect on the year as a whole, the manner of Nadal’s 6-3, 6-0, 6-1 destruction of Federer in the final at Roland Garros may be seen as a watershed. The win at Wimbledon four weeks later was a massive blow, the effect of which has been clear in the past two weeks.

First Federer was beaten by Gilles Simon, of France, in the second round of the Rogers Cup in Toronto. Then, last night, he fell under a barrage of 22 aces from Karlovic, a man who had taken just three sets from him in six previous meetings.

The loss of top spot would surely be hard for Federer to take. Pete Sampras’s record of 14 grand-slam titles had seemed his for the taking, but now who is to say he will even win another grand-slam title? Nadal seems to be improving each week and Novak Djokovic, the Australian Open champion, is a growing threat.

Federer has won only two titles this year, in Estoril and Halle, he lost in the semi-finals of the Australian Open and in addition to his defeats by Nadal in Paris and at Wimbledon, he has suffered several shocking losses. Now we will see what he is made of. If he rebounds, then he will probably be considered an even greater champion. If not, as strange as it seems, it could be the end of him as a grand-slam threat.

Federer’s defeat was Andy Murray’s gain as the British No 1 moved into the last eight last night with a 6-3, 6-3 win over Dmitry Tursunov, of Russia. A semi-final against Federer had been his goal at the start of the week, but his path is relatively clear. His next opponent will be either Igor Andreev, of Russia, or Carlos Moyà, of Spain. Maria Sharapova has pulled out of the Beijing Olympics after an MRI scan on her right shoulder revealed two small tears. The Russian world No 3 was examined after withdrawing from the Rogers Cup on Wednesday evening with a sore shoulder.

Handover of power

- Roger Federer has been the world No 1 for 235 consecutive weeks, since February 2, 2004

- Rafael Nadal has been world No 2 for 158 consecutive weeks

- At start of this week 300 points separated the two

- Nadal will become world No 1 on August 4 if he wins the event in Cincinnati

- The Spaniard could become No 1 on August 11, depending on the Cincinnati result – this is because Federer will lose 125 more points than Nadal once last year’s result in Toronto drops off the computer