MACAU (AFP) - Former tennis great Bjorn Borg on Tuesday backed Roger Federer to return to top form after a damaging slump which has left his number one ranking under threat. Federer was playing too well to relinquish the top spot, Borg said, although he stopped short of tipping the Swiss to win gold at next month's Olympics. "Nadal is getting closer - he beat Federer at Paris and Wimbledon - but I think Roger will come back," Borg said by telephone from Sweden. "I know Roger wants to win Grand Slam tournaments, he wants to stay number one in the world but Nadal is getting closer. It's going to be interesting to see what happens at the US Open." Nadal thrashed Federer for the French Open title before outlasting the 26-year-old Swiss in this month's classic Wimbledon final - which Borg described as the greatest he had ever seen. "The Wimbledon final was probably the best I've ever seen played. They played unbelievably for over five hours," Borg said. "To have that kind of rivalry they've been having for three, four years now is great for tennis - it's great for sports." Nadal won the Toronto Masters on Sunday to close to within 300 points of Federer in the ATP rankings, threatening his record-breaking stay at number one which goes back to February 2004. Six-time Grand Slam winner Boris Becker has already anointed the Spaniard as the world's best after he added the Wimbledon title to his four consecutive French Open crowns. "If you talk to anybody in the world of tennis, the number one is the winner of the French Open and Wimbledon," Becker said. Borg retired at 26, the same age as Federer, after also seeing his five-year Wimbledon winning streak ended at the hands of John McEnroe in 1981. But he said Federer, who has talked of extending his career into his 30s, retained the drive to keep winning. "I know Roger's very determined to win Grand Slam tournaments. He's still very motivated to break new records," Borg said. "Right now he's going to have a tough time but I think he's going to come back. Roger is still playing unbelievable tennis and he was very close to winning the Wimbledon final. "For me he's going to win many, many more Grand Slam titles. It's just a matter of keeping playing. He's not injured, he's healthy and he's motivated." Borg was speaking ahead of the Macau Tennis Showdown 2008, where he will play former nemesis McEnroe on November 20. Federer will face either Nadal or James Blake, depending on the result of the US-Spain Davis Cup semi-final in September. Borg regretted never having the chance to play at the Olympics, which only welcomed tennis back to the programme in 1988, and said the outcome of this year's tournament was impossible to predict. "I think Roger really wants to win but to pick a player is very difficult right now," Borg said. Borg, 52, won 11 Grand Slam titles before retiring in 1983 and embarking on a rollercoaster clothes business venture.