Yes, this match does feel very strange. During the 4th set, Borg looked to be more concerned about exiting than actually staying and fighting out a potentially long match. He seemed to be experiencing something bordering on anger, fear, and frustration.
You could see it in his shotmaking, which did make him look like something "alien" or like a "ghost" as Hoodgem mentioned. The death threat stuff is downright scary. In a big stadium like that, in that unique New York environment, with the added "shuffling" and slight chaos, versus say Wimbledon, it would be very distracting quite possibly. If I could ask him, I'd ask about the impact on him of that death threat at the US Open. Was that the "final straw" coupled with his fights with Tour Organizers as far as requiring 10 tournaments a year as a minimum in 1982 and for the foreseeable future?
Perhaps he just said, you know what, I've given enough of myself to this Game if that's how "they'll" treat me. I'm done and have decided to seize my life and take it from tennis. There is a burn out element to it and again, I reiterate, if he could have coupled a layoff with the ability to cut down on his tournament schedule to say 8 tourneys in 1982, 1983, as opposed to 10+, that was what he needed. Also, go in the lab with Donnay and get a pure graphite frame developed by say 1983 (try maybe 80 sq inches at least, just a thought??), as guys were making that switch, including McEnroe by 1983!
It's almost as if something dawned on Borg during the day before the match and then during the match, and he said, this death threat is just too much. I remember he went into the 1981 final, having beat Connors in STRAIGHT SETS, while McEnroe was coming into the final after a 5 setter with Gerulaitis. So, he was obviously in pretty good form after the SF. What suddenly changed? The death threat must have played a role here.
All in all, Borg needed a layoff after this match and a Tour Organizers that would allow him to possibly play less than 10 tournaments for 1982, without him HAVING TO QUALIFY for say the 1983 Grand Slams. UNIMAGINABLE!
This link has a bit more footage including the end. I can't help thinking that at about 9:20 in this video, he's thinking about leaving the Stadium with his health, given the threat.
Anyway, see these excerpts from Wikipedia:
Borg went on to lose to McEnroe at the 1981 US Open, 4-6, 6-2, 6-4, 6-3, and the defeat effectively ended Borg's career. After that defeat, Borg walked off court and out of the stadium before the ceremonies and press conference had begun. It would turn out to be the Swede's last Grand Slam final. Although he felt in good condition physically
, he recognized that the relentless drive to win and defy tour organizers had begun to fade.
In 1982, Borg played only one tournament, losing to Yannick Noah in the quarterfinals of Monte Carlo. Nevertheless, Borg's announcement in January 1983 that he was retiring from the game at the age of 26 was a shock to the tennis world. McEnroe tried unsuccessfully to persuade Borg to continue.