Ivan Lendl on Wednesday played down Andy Murray's chances of sealing back-to-back Grand Slams at the Australian Open, warning it was "very, very difficult to succeed" in a highly competitive era.

Lendl has coached Murray to Olympic gold and the US Open title, ending Britain's 76-year wait for a men's Grand Slam singles win, but he said it would be tough in January with the world's top four all expected to be in action.

The Czech-American great was speaking by teleconference at an announcement in Hong Kong that he will play John McEnroe, and China's Li Na will face Danish star Caroline Wozniacki, at an exhibition event in March.

"Tennis is a very difficult sport to win these days with the likes of (Novak) Djokovic, (Roger) Federer and (Rafael) Nadal and Andy in that group -- it's very, very difficult to succeed," Lendl said, when asked about Murray's chances in Melbourne.

French Open champion Nadal missed much of the season just ended with a knee injury, easing the task of his rivals, but is now back in training.

Djokovic won this year's six-hour Australian Open final against Nadal, and this month he dominated Federer to clinch the season-ending ATP Tour Finals.

Meanwhile Federer, 31, beat Murray to bring up a record-equalling seventh Wimbledon title and return to world number one.

With Murray's win in September, the year's Grand Slams were shared among the top four players. But Lendl, whose rivalry with McEnroe was legendary, was cautious about whether the current era was the greatest seen in men's tennis.

"I think you can look at many eras in the game and say there have been many good players at the same time," he said. "You can look at the early Eighties with Connors, McEnroe, Bjorg and myself, and compare it to the Sixties and early Seventies with all the Australians there.

"I don't think you should be comparing one era to the other because it's just not comparable."

Lendl, a former world number one and winner of eight Grand Slam titles, will soon mark a year of collaboration with world number three Murray, his first coaching position. But he refused to be drawn on their targets for next season.

"I am not going to go there... I never get into the details of his game with anyone because it not necessary for anyone other than Andy to know," Lendl said. "The best way to go about it is one game at a time."

However, he said the key to motivating Murray was setting and achieving high goals, and added that their relationship worked because they both found it "fun".

"Andy needs to motivate himself like every other player. He needs to set goals and then try to achieve those goals. They need to be high but realistic and once you achieve those goals, then that's your motivation," said Lendl.

He added: "I think having a good relationship, enjoying each other's company while working together makes it easier because the work is very hard for Andy. If it's fun, it's easier to do."

And Lendl also admitted that if had played Murray while still in his prime, there would only be one winner.

"He would kill me. All you have to do is look at the sports where you can measure times -- look at swimming, look at track and field and compare the times you had 30 years ago to the times today," Lendl said.

Lendl will reprise his rivalry with McEnroe and Li will play Wozniacki in Hong Kong on March 4, at the exhibition event sponsored by French bank BNP Paribas for the International Tennis Federation's inaugural World Tennis Day.