Roger Federer will face a "reality check" when he returns to the Tour after a lengthy injury break, seven-time major champion Mats Wilander said.
Federer, 40, told Swiss media last week that he was unlikely to return to competition until mid-2022 as he looks to get back to full fitness after multiple knee surgeries and would be "extremely surprised" if he was able to play at Wimbledon.
Federer, who is tied with Rafa Nadal and Novak Djokovic on 20 Grand Slam singles titles, said he would be able to resume running in January and return to on-court training in March or April.
"I think it's possible for him to come back," Wilander told Eurosport. "I think if he keeps saying it himself, he obviously has his mind set on coming back.
"I think the bigger problem to me is that these guys are getting really, really good. (Alexander) Zverev and (Daniil) Medvedev are the guys pushing behind, they are playing huge games of tennis.
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"And of course, Federer has his serve but how big a weapon is a Roger Federer serve compared to let's say a Zverev or Medvedev serve? I think that's going to be the reality check."
Federer had two knee operations in 2020 that kept him out of the Tour for more than a year and returned to action in March but managed to play only 13 matches this year.
Another injury at Wimbledon forced Federer, who won his last major at the 2018 Australian Open, to skip the US Open as he went under the knife again.
"I don't think that Roger Federer will play too many matches on Tour," Wilander said.
"He realises that he does still have a chance to win -- maybe not a Grand Slam, maybe not a final -- but three or four matches. I think it's worth it for him to come back in his own mind.
"I don't think he will know until he steps on court for at least a two- or three-set match against the likes of Zverev and Medvedev."