McEnroe, after all, ended Borg's streak of five consecutive Wimbledon titles by beating him in the 1981 final. McEnroe also defeated Borg twice in U.S. Open finals, and is widely credited with hastening the Swede's early exit from tennis.
And yet the two men stay in touch regularly and have what Borg calls a "nice relationship." Indeed, he's hoping they'll face each other on a court this week, when the 53-year-old Swede will be playing competitive tennis in the United States for the first time in a decade.
Borg is participating in the 30-and-over Outback Champions Series event in Boston that begins Thursday with eight entrants. If Borg beats Mikael Pernfors in the opening round, he'll meet the winner of a match between McEnroe and Mats Wilander.
"With all the great matches we had in the past, it's always something special to see John's face on the other side of the net," Borg said by phone from his home in Stockholm.
Borg said he respected McEnroe — on and off the court — more than any other player of their generation.
"I think he feels the same way, too," Borg said.
He won 11 Grand Slam singles titles from 1974 to 1981 — five at Wimbledon, and a record six at the French Open. Borg and McEnroe played each other 14 times on tour, splitting the career series.
Their most famous match was the 1980 Wimbledon final, in which McEnroe won a fourth-set tiebreaker 18-16, but Borg won the fifth set 8-6. It's among the top matches in tennis history, replayed on TV year after year during rain delays at the All England Club.
Borg has watched it in its entirety only once, though.
"It's very rare that I see myself, that I watch my old matches on tape. Very rare," Borg said. "It feels a bit strange to watch yourself. I don't really like that."
But about 15 years ago, his son Robin, who was 10 at the time, asked if they could pop in a tape of that Wimbledon final.
"We both played great. Great, great match," Borg said. "To sit through the whole match and watch every, single point for five sets — that was the first and only time I've seen the whole match."
Borg and McEnroe have become close in recent years, and the American was one of the main reasons Borg decided to sign up for this week.
"He has been trying to convince me to come for the last two or three years," Borg said.
Also pushing him? Jim Courier, the former No. 1 and four-time Grand Slam singles champion who founded the Champions Series.
"I've been trying to entice Bjorn to play in one of our events for a long time, and he plays very sparingly these days," Courier said. "I must have finally worn him and his agent down with my persistence."