Greg Rusedski has warned Andy Murray still has a long road ahead if he is to return to the top of the game.
The former world number one has surprised many, including Rusedski, with the level at which he has been able to play only eight and a half months after hip resurfacing surgery.
In a three-week stint in China, Murray beat now world number 11 Matteo Berrettini and pushed Dominic Thiem and Fabio Fognini hard in close defeats.
It was a big step up from the performances Murray produced on his return to singles action in America in August and gave hope to his fans, as well as the Scot, that he could perhaps again challenge for tennis' biggest titles.
But, after two years of struggles with his right hip and at the age of 32, Rusedski believes it is too early to make future predictions.
The former British number one told the PA News Agency: "I've been pleasantly surprised. I thought it would be much more difficult for him, I thought it would take a little bit longer, but he's played very, very well.
"The question mark is still how much can his body take week in, week out. There's still a long road back.
"He's played two matches back to back, but can he play three, four, five days in a row? It's too hard to predict. I think we'll know more when he plays the Australian Open and a three-out-of-five-set match.
"It's not his mentality or his tennis, it's what the body will allow him to do. But the signs are very positive."
Rusedski and Murray are on a list that now includes Dan Evans, who at 43 in the world is the top-ranked player in Britain for the first time.
Evans' stellar form since returning unranked from a year-long drugs ban last spring coupled with Kyle Edmund's slide has left the 29-year-old in top spot, and Rusedski believes he can climb significantly higher.
"Sometimes you go through difficult situations and it forces you to grow up and take stock and do all those things that Dan has done," said Rusedski.
"We've always known he had the talent and the ability and I had a feeling at the beginning of this year when I saw him in Australia that he was going to have a fantastic season.
"I think top 20 would be a realistic goal for him if he can keep on the momentum he's had this season."
Edmund will find himself outside the top 70 on Monday having started the season ranked 14.
A knee injury plagued him for the first half of the season and Edmund is currently on a seven-match losing run dating back to early August.
Rusedski also cited the split from coach Fredrik Rosengren, who retired abruptly in February.
"He's got to reset, look at what he needs to do and really use this off season to push forward and improve his game," said Rusedski, a former coach of Edmund.
"It's hard to get up to the top 15 but it's even harder to stay there because you've got to continually get better, you have to be healthy and you have to have the right team around you.
"The second year when you get there is always the toughest, which Kyle's gone through, and year three is usually the rebound year so there's no reason why Kyle shouldn't be back in the top 50 in 2020."
Rusedski was speaking while he helped out at a Cancer Research UK shop to launch the charity's partnership with the Nitto ATP Finals in London next month.
A treasure hunt across several Cancer Research UK shops in London and on the charity's EBay shop from October 18 to November 10 will offer fans the chance to claim prizes including signed items from leading players.
Rusedski said: "Everybody in their life has been affected with cancer somehow or another, so I think it's a great cause and a great partnership."
The tournament at the O2 is shaping up to be a battle of the young challengers versus the old guard, with US Open finalist Daniil Medvedev the current front runner to be the best of the next generation.
Medvedev's success has reignited the debate about when a new face is going to win a grand slam.
"It's getting closer," said Rusedski. "I thought it was either going to be next year or the year after. Medvedev is the guy you're looking at who could possibly win one. It'll be a very exciting end of the season, which will set up 2020 beautifully."
:: Keen treasure hunters are advised to seek out @CRUKshops on Instagram to get involved. For tickets to the Nitto ATP Finals at The O2 visit: www.NittoATPFinals.com/tickets