Federer is currently second behind Connors in the list with 1,108, ahead of Ivan Lendl on 1,071.
The American great is out on his own with 1,243 across a glittering 25-year career.
Federer has had a stunning 2017 in the tournaments he has played, winning the Australian Open and Wimbledon.
But he had an extended break during the clay-court season, dramatically reducing his chances of hunting down Connors.
Former British No 1 Croft says the lack of tennis Federer, 36, is playing means it is going to be tough.
“It’s going to be quite difficult for him,” she said.
“We just don’t know how long that Roger Federer is going to keep going.
“Although judging by this year, you look at what he’s achieved this year.
“He’s not played a lot of events, perhaps nine events he’s played this year and he’s done an awful lot of winning in those events.
“But he hasn’t overplayed, so I think it will be tough for him to get beyond the top of the leaderboard there.”
Federer will look to add to his win tally this afternoon when he faces Richard Gasquet at the Shanghai Masters (1pm).