Serbian Novak Djokovic's U.S Open title allowed him to clinch the year-end No. 1 ranking for the fourth time on Monday.The ATP announced Monday, a day after Djokovic's 6-4, 5-7, 6-4, 6-4 victory over No. 2 Roger Federer in the final at Flushing Meadows, that the 28-year-old would add 2015 to 2011, 2012 and 2014 as seasons he finished atop the rankings."Knowing I will end the year at No. 1 keeps my mind relaxed," Djokovic said in an interview with The Associated Press. "I have achieved a lot so far in the season, and I hope I can deliver the same game for the rest of the year."He is 63-5 with seven titles, including three at Grand Slam tournaments, and has reached the final at each of his past 11 events. Djokovic beat Andy Murray in the Australian Open final, lost to Stan Wawrinka in the French Open final, then beat Federer in the finals at Wimbledon and the U.S. Open.Djokovic is currently in his third stay at No. 1, for a total of 164 weeks, the sixth most in the history of the computer rankings that began in 1973. Federer holds the record of 302.There wasn't much movement at the top of the ATP rankings Monday, although Marin Cilic, last year's U.S. Open champion, went from No. 9 to No. 14 after losing to Djokovic in the semifinals. Kei Nishikori, the 2014 runner-up, slid two spots to No. 6 after a first-round exit at Flushing Meadows.The two Italian women who met in the surprising U.S. Open final, champion Flavia Pennetta and runner-up Roberta Vinci, both made big jumps in the WTA rankings.Pennetta rose 18 spots to a career-best No. 8 on Monday, two days after beating Vinci to become, at 33, the oldest first-time Grand Slam champion. Vinci, who stunned Serena Williams in the semifinals, went from 43rd to 19th.Williams remained at No. 1 like Djokovic, she already is assured of finishing the year in the top spot followed by No. 2 Simona Halep, No. 3 Maria Sharapova and No. 4 Petra Kvitova.