Quotes from ATP website:
Battle For No. 1
If there is no change at the top of the South African Airways ATP Rankings in Cincinnati, a change could come August 18 - two weeks later - due to a calendar shift this season to accommodate the Olympics.
Last year, ATP Masters Series events in Canada and Cincinnati were played two weeks later in the season than they are being played this year. Because points earned by players count towards their rankings for 52 weeks, the 850 points Federer earned last year by reaching the Canada final and winning the Cincinnati title will continue to count towards his ranking during the same Masters Series tournaments this year.
Come August 18, those 850 points will no longer count towards Federer's ranking. In contrast, Nadal will drop 230 points (225 points for reaching the Rogers Cup semifinals in 2007 and five points for losing his first match in Cincinnati in 2007.)
Federer and Nadal have been No. 1 and 2 in the South African Airways ATP Rankings every week since July 25, 2005 when Nadal became No. 2. Federer has ranked a record 234 consecutive weeks at No. 1 (since Feb. 2, 2004) and Nadal for 157 straight weeks at No. 2.
Nadal, who has ranked No. 2 for a record 157 weeks, will become the new No. 1 next Sunday if he wins the title at the Western & Southern Financial Group Masters in Cincinnati and Federer loses before the semifinals. After Federer’s second round loss to Gilles Simon earlier this week, Nadal has now cut the gap to a mere 300 points and is potentially just one week away from ending Federer’s 234 week reign (since February 2004) at the top spot (see below).
If Nadal gains 300 or more points than Federer in this year's Cinci then number 1 is his next week. If not then last year's points from Canada and Cinci MS will be deducted on August 18 giving a 620 point deficit to Federer compared to Nadal.
Seems like Federer's time at number 1 is almost over