Australian Open: It's game on at the top with no Rafael Nadal For Rafael Nadal, however, the wait to resume competition following the knee injury which has kept him off court since Wimbledon continues. The Spaniard yesterday announced his withdrawal from both the year's opening Grand Slam tournament and next week's Qatar Open. He blamed the stomach infection which forced him to pull out of this week's exhibition event in Abu Dhabi and has disrupted his recovery programme. The news will increase speculation as to whether Nadal will ever recapture the fitness that his all-action game style demands. It also threatens to change permanently the landscape at the top of men's tennis, which for the last three years has been dominated by Nadal, Roger Federer, Novak Djokovic and Andy Murray. While Federer will not play anywhere before the Australian Open, Murray and Djokovic both compete next week. Murray is the top seed at the Brisbane International, which begins tomorrow and also features the world's top three women, Victoria Azarenka, Maria Sharapova and Serena Williams. Djokovic, who beat David Ferrer in his opening match in Abu Dhabi yesterday and meets Nicolas Almagro in today's final, is playing at the Hopman Cup in Perth, which was due to start this morning. The field at the mixed team event also includes Ana Ivanovic, Venus Williams and Jo-Wilfried Tsonga. The men's tournaments in Doha and Chennai begin on Monday, while there are women's events next week at Shenzhen in China, where Laura Robson makes her season's debut, and at Auckland in New Zealand, where Heather Watson will be playing. Novak Djokovic (Serbia, aged 25, world no 1) For the last two years Novak Djokovic has been the most complete player in men's tennis. Having previously had question marks over his stamina, his serve and his net game, the Serb has become a superb all-rounder, a phenomenal athlete with no weaknesses. At 25, the world No 1 is in his prime and looks capable of adding several more Grand Slam titles to his current tally of five. Younger rivals are struggling to break into the top group, while there are good reasons to doubt whether Roger Federer and Rafael Nadal, for reasons of age and fitness respectively, can stay with the pace Djokovic has been setting, even if Andy Murray can be expected to sustain his challenge to the Serb for years to come. Three of Djokovic's five Grand Slam titles have been won at the Australian Open, where he is the short-priced favourite to win again next month after finishing 2012 in stunning form. Between the Olympics and the end of the season Djokovic (right) lost only three matches and won titles in Toronto, Beijing and Shanghai before rounding off the year with victory in the Barclays ATP World Tour Finals in London. As he looked ahead to 2013, Djokovic described his triumph at the O2 Arena as "a strong wind in my back." He added: "It's going to help me to get better on the court, to try to win as many big tournaments as possible. My goals, ambitions and expectations for myself are always very high. The new year could provide Djokovic with the chance to move alongside Nadal, Federer, Don Budge, Fred Perry, Rod Laver, Roy Emerson and Andre Agassi, who are the only men to have won all four Grand Slams. The French Open is the one major to have eluded Djokovic so far. Much will depend on Nadal, who has not played since Wimbledon because of knee trouble and has again delayed his return to competition. If the king of clay is operating below 100 per cent at Roland Garros, Djokovic could take advantage. Roger Federer (Switzerland, aged 31, world no 2) No modern player has organised his training programme and tournament schedule more effectively than Roger Federer, which makes his planning for 2013 all the more intriguing. The 31-year-old Swiss (left) who has a superb fitness record, has pencilled in a break of nearly two months between the Indian Wells Masters in March and the Madrid Masters in May. Federer will miss both the Miami Masters (for the first time since his debut there in 1999) and the Monte Carlo Masters, which is the traditional start of the European clay-court season. After a busy period following the Australian Open (he will play in Rotterdam, Dubai and Indian Wells), Federer will use his spring break to work on his game in preparation for the climax of the clay-court campaign and the grass-court season. "I need to practise a lot more next year," Federer said. This year I hardly had an opportunity to do that. Winning an eighth Wimbledon singles title, which would take him clear of William Renshaw and Pete Sampras, with whom he shares the current record, is likely to be Federer's main goal. In 2012 he had the additional ambition of reclaiming the world No 1 ranking - which he achieved - but a more limited schedule in 2013 would make that a tough mountain to climb again. While the likes of Andy Murray were running themselves into the ground at training camps earlier this month, Federer played exhibition matches in South America on a trip which was reported to have earned him $12m (about £7.4m). He will not appear again before the Australian Open, instead training at his warm-weather base in Dubai. It might not seem the ideal preparation for the year's opening Grand Slam tournament, but he has won the Melbourne title before without making any competitive appearances in the build-up. Federer rejected any suggestion his off-season programme might have jeopardised his chances Down Under, pointing out that he has not entered any exhibition events between Christmas and the Australian Open. "Basically I've given myself enough space," he said. It's just about making sure you manage your schedule correctly. Rafael Nadal (Spain, aged 26, world no 4) For the last six months Rafael Nadal has clung on to his place in the world's top four thanks to his outstanding results in the first half of this year. From next month, however, he will start to tumble down the rankings. David Ferrer is certain to overtake his fellow Spaniard at the end of next month's Australian Open, and others could also follow suit. If Nadal is not firing on all cylinders by the start of the European clay-court season in April, where he won the vast majority of his current rankings points earlier this year, his slide down the world order will be spectacular. For the moment, nevertheless, the concern is less about Nadal's ranking and more about whether he will ever be able to play again at the highest level. His immensely physical game style puts great stress on his knees. Hard courts in particular take their toll, but what was most worrying for Nadal was the fact that the problem flared up again this summer at the end of a three-month period during which he had been competing only on clay and grass. Nadal has been advised to rest for the next week and is now targeting a tournament in Acapulco in February for his return. "My knee is much better and the rehabilitation process has gone as well as predicted by the doctors, but this virus didn't allow me to practise this past week," he said yesterday. As my team and doctors say, the safest thing to do is to do things well and this virus has delayed my plans of playing these weeks. I always said that my return to competition will be when I am in the right conditions to play. After all this time away from the courts I'd rather not accelerate the comeback. I prefer to do things well. Nadal and his entourage will not be the only ones keeping their fingers crossed that he can make a successful comeback. The Spaniard's energy and charisma were sorely missed in the latter half of 2012 and reinforced the fact that he and Federer remain the game's two biggest draws. Andy Murray (Great Britain, aged 25, world no 3) One of the most frequent complaints of players is a lack of time to prepare for tournaments, but Andy Murray should not have any such gripes come the spring. When he heads for the Indian Wells Masters in March, Murray is likely to have played just two tournaments in the preceding four months. This year's busy calendar, which included the Olympics, gave Murray few opportunities for quality time on the practice court with his coach, Ivan Lendl. However, the mid-November finish to the 2012 season enabled the Scot to take a holiday and still spend a month training in Miami. After playing next month in Brisbane and the Australian Open in Melbourne, he plans another training camp in Florida with Lendl before returning at Indian Wells. "It's been great having Ivan around," Murray said of his recent spell in Miami. It's the longest time I've worked with him on a training block. It's such an important time for me and sets me up nicely for 2013 and the Australian Open. I've been spending plenty of time on the court over the last few weeks, working on specific drills and fitness. It's nice to be able to do that for an extended time away from the tour. The next six months might be Murray's best chance of becoming world No 1, though he needs to be more consistent away from the Grand Slams, particularly after failing to win a Masters Series title this year for the first time since 2007. The world rankings are based on a rolling 12-month total of points earned. Murray (8,000) currently trails Novak Djokovic (12,920) and Roger Federer (10,265), but he has comparatively few points to defend before Wimbledon. However, interest in Murray is sure to focus on Wimbledon, especially after he proved himself at the top level with wins at the Olympics and US Open. After becoming the first British man to reach the singles final at the All England Club for 74 years this summer, the nation will be hoping he can end a 77-year wait for a home champion in 2013.