Rafael Nadal has shown good progress in training since recovering from a hip injury but his coach Carlos Moya still worries about how the former world number one's body will hold up against the rigours of Grand Slam tennis.
The 37-year-old suffered a muscle issue at Melbourne Park in January that wiped out his season but will return at Brisbane this month before playing in the January 14-28 Australian Open where he won two of his 22 Grand Slam titles.
The Spaniard, who had surgery in June, has said 2024 could be his final year on tour and has stepped up training in recent months in order to make a comeback and ensure he finishes his career on court.
"Rafa's going to go from training, which he's doing very well, to competing. It's impossible to have the same conditions in training as in a match," Moya said in an interview with the ATP Tour.
"Playing the best of five sets, win, rest, return to court two days later ... That's the doubt I have right now, especially for a Grand Slam. But we have time. If the Australian Open started tomorrow, it would be a real fear.
"There's still a month left, a tournament before in Brisbane, demanding training ... I think all of this will put him in a position to be able to endure it. But now that's my fear."
Nadal dropped out of the top 100 for the first time in 20 years earlier this season and has slipped to number 668 but is eligible for a protected ranking having been injured and not competing for at least six months.
However, Moya said Nadal would not be able to avoid strong opponents in the early rounds and would need favourable draws to build up his rhythm.
"I've never been of the thought that Rafa needs rhythm because he's too good for that," Moya said. "But now the situation is different."
