This isn't just cases someone would have almost for sure beaten someone else in the final or the round they would have met given the match up. What are times someone was taken out (usually in a huge upset) and that player would have clearly reached the end or atleast this player and prevented them from winning otherwise, would not have been stopped in any other round either. Here are some fairly obvious cases to me.
1998 French- Sanchez Vicario, Seles beating Hingis. Seles beat Hingis in the semis in a pretty major upset as she had a 0-5 record vs Hingis going into that match. Even more amazing it was a pretty easy victory. Then in another pretty big upset, Seles fell to Sanchez Vicario in the final, even after her excellent semi final performance, and with her career I think it was 14-2 record vs Sanchezz at the time. Now I know it sounds like a contradiction to say Hingis would have for sure beaten Sanchez with her overwhelming head to head over her, given that I just listed two matches the player who had a lopsided losing record vs the other won, so why not another, LOL! For the record Hingis is 18-2 vs Sanchez lifetime, with 1 of the 2 losses being a 3 setter as a lowly ranked 15 year old. However to say anyone who has actually watched Hingis play Sanchez Vicario, you would know she was never beating her in such a big match at this stage of their career. It was a far worse and more hopeless match up for Sanchez than either Sanchez vs Seles, or Seles vs Hingis was, both which had a lot of deceptively close matches in some of the losses. As anyone who follows Hingis knows the only way to possibly beat her is to overpower her or attack her (Novotna), and Sanchez ain't ever going to do either one to Hingis. So not even on clay, not even at Roland Garros where Hingis seemed jinxed her whole career, and not even when Hingis was in arguably sort of bad form at the time. Just would never happen. If Seles did not take out Hingis for Sanchez Vicario, there is no chance at all Sanchez Vicario wins this RG, as she stands no chance to beat even a subpar Martina Hingis in the final. I would say if Seles lost in any round, but then again it is possible someone like Novotna or whoever takes out Hingis, and Sanchez beats them in the final, so specifically just if Seles loses to Hingis in the semis. The one thing we know for sure is Sanchez is the last one in the entire draw to beat Hingis probably of the round of 32 beyond.
1987 Wimbledon- Pat Cash, Peter Doohan beating Boris Becker. Becker seemed in excellent form. He absolutely owns Cash. This was one of the biggest upsets in Wimbledon history at the time. Looking through the draw I don't see anyway Becker does not reach Cash. Who is going to take him out,
Slobodan Živojinović, way past his prime Jimmy Connors? I would say hardly even a miniscule chance, Becker is taken out before Cash if he doesn't lose in that mammoth upset to Doohan. And yeah I know some will say will say neither of those would be as much of an upset as Doohan, but lightning of that magnitude was only meant to strike once that event. And I would argue in fact Doohan is more likely to pull an upset of Becker since he is an unknown, while Becker played Zivojinovic 5 times and his 1 loss was in a match he retired in the match with a major injury, and played old Connors 6 times and won all 6, and this is now on grass, Becker's favorite surface. As for Becker vs Cash, well Cash has only win, 7-6 the final set, in Australia, on a hard court. Their other 3 matches were on grass, 2 at Wimbledon, including the very next year. All very easy Becker wins. I would say while Cash would have an outside shot, Becker would be the heavy favorite in a hypothetical semi final.
1990 Wimbledon- Zina Garrison, Martina Navratilova, Monica Seles, Steffi Graf. This ring sort of involves 4 players. Anyway Zina Garrison had a match point vs Seles in the quarters. If she converts, it is a Graf vs Seles semi final, which is a guaranteed Graf victory (Graf won 6-2, 6-1 and 6-1, 6-0 in their only 2 meetings at Wimbledon). Or if Zina does not take out Graf herself in the semis in a gigantic McNeil 94 level upset. Either of these 2 scenarios leads to a Graf vs Navratilova final. Which while it is not impossible Navratilova wins I very much doubt she does. She lost to Graf at both Wimbledon 88 and 89, and while both went to 3 sets, both were highly decisive wins in the end with a very lopsided 3rd set, and all 4 sets Graf won were easy sets. Even with Graf in subpar form at the time, and Martina in fact playing really well at this Wimbledon, I don't see a 33 year old Navratilova on bad knees stemming the tide at this point. Graf had only lost 1 of the previous 5 slams at that point too (the RG final vs Seles, and her last loss before that was the RG final to Sanchez a whole year before that) so her aura was not gone yet at that point, despite her already visibly diminishing play.
2009 French Open- Rafael Nadal, Robin Soderling, Roger Federer. This is one of the easiest ones ever. Both are familiar enough to everyone, I am sure everyone, even the biggest Federer diehards concede there is no way on gods green earth Roger beats Nadal in a hypothetical RG final. Nor is there anyway that Gonzalez or Davydenko (yes Davydenko has a decent record vs Nadal, on hard courts, this is clay and RG) is taking out Nadal, if Soderling does not do it.
1994 Wimbledon- Conchita Martinez, Lori McNeil, Steffi Gaf. Honestly this one is as easy as the 2009 French Open. Lori McNeil took out Steffi Graf in the 1st round of Wimbledon 94 in one of the biggest upsets ever. Obviously going through McNeil's draw there is no way Graf is losing to anyone else en route to Conchita Martinez in the semis. Nor is there anyway Conchita Martinez, with a lifetime 1-12 record vs Graf, and the big girls favorite whooping girl in slams, beating Graf on grass in the semis. She couldn't even do it vs Graf on clay 11 months later, coming in the best form of her career, totally dominating the clay circuit, and Graf missing the entire clay season injured/ill.
1994 Wimbledon- Conchita Martinez, Martina Navratilova, Jana Novotna. Another one from Conchita's Wimbledon win. Navratilova took out Novotna in a 3 set quarter final. Navratilova of course went on to lose to Conchita in the final. I am comfortable saying there is virtually no chance Novotna loses either Gigi Fernandez in the semis, or Conchita Martinez on grass in the final. Their career head to head is 4-1 Novotna, and this is grass, Jana's best surface by far and Conchita's worst (despite the massive, bizarre, fluke that her Wimbledon title was). Even if Chokevotna showed up at some point, she would still seal the win. Elder Navratilova owns Novotna (probably mostly mental, also Jana can't pass off the backhand side at all so struggles vs other serve and volleyers, even a way inferior player like Tauziat for instance), and Martinez owns elder Navratilova, so the results of those two match ups were no surprise, and very favorable for Martinez, as she avoided another player basically certain to have won over her.
2002 Australian Open- Jennifer Capriati, Monica Seles, Venus Williams. Seles scored her only win over Venus here, in a 3 set quarter final. Now it is worth noting when assessing this whole situation Venus did have a leg injury, although it wasn't as severe by the quarter final defeat as it was a few rounds earlier when it incurred, it was still present. Seles went on to lose to Hingis in the semis, and Hingis went on to lose to Capriati in the final, after having 4 match points in the 2nd set. Even with that, and their very competitive head to head history, I would say there is almost no chance she loses to declining/lost all confidence in big slam matches Hingis in the semis. And no way IMO she loses to Capriati in the final, when Jen is her pigeon (4-0 career record) and Capriati was not even playing well at this event. She played not only much better in the 2 slams she won, by a whole horde of one she didn't win in, particularly in 2001-2004. She basically won due to amazing fight, and Hingis's gigantic mental block in slams at this point; another reason I am pretty sure Venus beats Hingis in the semis as well. I just don't see Venus losing here if Seles does not score her only ever win over her in the semis, even with the legitimately present injury, even with her virtually tied career head to head with Hingis.
There are many other possible ones but these are some of the most obvious ones of all time that come to think for me. Will think of some other super obvious (for me) ones to post later. Come up with your own and let me know which of mine you agree with or some thoughts on them.