That's one I don't remember. Can you post a link to an article, or scan one with the evidence? Because I find it very difficult to believe that a 1979 Roscoe would lose to Martina in that time period. Martina had not reached her zenith, and Roscoe still had a serve. Something had to be up with that...
I think the source for this was a newsgroup, so you might be able to google it. (Not 100% sure though. My data accumulation often largely consisted of gathering the info itself. My results pages, for example, have been culled from literally thousands of books, magazines, and newspapers over the past quarter century plus, from many libraries, and space was at a premium.) It was a while back, though, so you might just wind up with a reference to it instead of the original. You could start with that one that used to post tennis news all the time (draws and results), whose name I can't recall offhand. (The new MSNTV browser doesn't support newsgroups, so I don't have access to all my old sources.) I think it had a ".ca" extension. Anyone know which one I'm refering to? (And thereby saving me trying to find it among thousands of pages of tennis information.

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My specualtion: Roscoe probably had a lot of early fault points lost, and had to resort to spinning the ball in. With one serve, it would largely negate his biggest weapon. And, let's face it, if Roscoe got into a rally, he was fair game, and that's just covering a normal court. And unless my memory is spotty, Roscoe wasn't a really fast runner around the court, by ATP standards of the day. And Martina's speed was one the most overlooked weapons she had. Even if he was a bit faster than her, it would be barely. When you weigh it as his speed covering the doubles court vs hers covering the singles court, that's a pretty big edge to Martina. And she was playing, and winning, a lot of mixed doubles matches, so returning a 3/4 speed or less Tanner serve wouldn't be a huge deal.
Again, this is just speculation, based on their demonstrated abilities, as I did not get to see the match itself. Apparently it wasn't considered a big deal or a huge surprise, or it might have been one of those matches that was just thrown together at the spur of the moment.
I do remember distinctly the remark from Connors, saying he couldn't even give his son the doubles alleys, when that proposal was made. It underscored his intense competitiveness. He just did not want to lose, even in an exo with handicaps against him, he wanted a realistic chance. Ironically, Martina hit maybe 2 shots the whole night into that extra space, so she wasn't capitalizing. She was very tight. (And not the good "tight" that you hear in recent slang.)
Volcana's remarks on older mixed singles matches reminded me of one between Tilden and Lenglen in the early 20's. (Reference to this is from Tilden's book, "Match Play and the Spin of the Ball".) To show how huge the gap was back then, Tilden says he played a practice set against Lenglen, and spotted her a 40-0 lead in every game, and still won 6-0.
Edit: remembered the name of one newsgroup last night-- clari.net. As an old orchestra member (who briefly played the instrument in question), I always used to chuckle a bit when I typed in the URL.