LONDON (AP) -- It was another easy win for Novak Djokovic, but certainly one that he will remember.
The Serb's celebrations at the O2 Arena after Tomas Berdych sent a final shot wide on Friday spoke volumes. Djokovic raised his arms above his head and gave a roar of joy, a large grin across his face. His comfortable 6-2, 6-2, win over Berdych had just sealed another year-end No. 1 ranking and a semifinals berth in the ATP Finals.
''Being No. 1 of the world is the pinnacle of the sport,'' said the 27-year-old Djokovic.
''One of the highest and most difficult challenges is to be No. 1 nowadays, especially because the tennis has become very competitive in the last five, ten years, very physical. To have a shot at No. 1 of the world, you need to be consistently healthy and successful throughout the year.''
Like Marin Cilic and Stan Wawrinka earlier this week, Berdych was dispatched with minimal fuss, giving Djokovic an unassailable points lead over second-ranked Roger Federer.
Berdych dropped his first two service games as the Serb took control from the baseline. Djokovic again broke twice in the second set with a range of forehand winners, powerful returns and net charges that frequently left Berdych stranded.
Djokovic, the Wimbledon champion, will finish the season No. 1 for the third time in four years. He's the seventh man to accomplish the feat at least three times.
''I believe everything happens for a reason. So I'm here for a reason, and I managed to finish the year as No. 1 for a reason,'' he said.
Djokovic and Federer have swept their groups in London, and are on course to meet in Sunday's final. After enduring one of his worst seasons last year, Federer is back to his best and proved to be a worthy challenger in their battle for supremacy.
''Roger is playing some of his best tennis since I know him and since I watch him,'' said Djokovic. ''We played an amazing final of Wimbledon. We fought all the way through basically the last few days of the season for No. 1. That proves how good he played over the course of 12 months. ''
Before a mouth-watering potential showdown, Djokovic will first take on Kei Nishikori in the semifinals, while Federer will face his Davis Cup teammate Wawrinka, who defeated U.S. Open champion Marin Cilic 6-3, 4-6, 6-3.
Djokovic has been in supreme form since the birth of his first child, Stefan, on Oct. 21, and has won 16 of his last 17 matches. In London, he extended his two-year-old indoor winning streak to 30 matches and has lost only nine games this week.
''I think I had a great season and a great year overall for my private life and professional, as well,'' he said. ''Considering all the things that have happened and circumstances, I thought I have done very well. ''
Besides winning his seventh Grand Slam title at the All England Club, Djokovic was also runner-up at the French Open, and won six titles.
Before taking some time off, he now has two more matches to win to claim a fourth title at the Finals and become the first player since Ivan Lendl from 1985-87 to win the elite tournament three years in a row.
Because of the formula used to determine the group standings, Wawrinka needed only to win three games against Cilic to finish second in his group and book a spot in the semifinals for the second consecutive year.
The Australian Open champion set about ensuring an all-Swiss semifinal when he raced into a 3-1 lead in the first set. But Cilic, who ended his maiden campaign in London with a third defeat, did not give up easily.
Cilic broke Wawrinka at love in the 10th game of the second set to even the score and built on the momentum to move 2-0 up in the decider.
But the hard-hitting Croatian failed to hold press home his advantage, allowing Wawrinka to break back. Cilic later dropped his serve following three consecutive unforced errors to hand his opponent a 4-3 lead.
Cilic saved one match point on his next service game but hit a double-fault on the second.