I don't think he meant that Graf personally had Seles stabbed. I understand what he meant. Had Seles not been stabbed she certainly would have won many more of those slams that went to Steffi. When Monica was taken out, she owned Graf. I would even say that the GOAT if not Serena would be Martina Navratilova.
I wouldn't say that Seles owned Graf before the stabbing. In 1990, Seles was 2-0 against Graf but in 1991 when Seles was #1, Graf was 2-0. In 1992, they were 1-1. And in the one match they played in 1993, Seles won it in 3 sets.
However, Monica definitely did dominate the tour that Steffi was a part of. And that's because Monica almost always brought her A-level stuff at the majors whereas Graf was often faltering in earlier rounds in 91 and 92.
IMO, one issue with Steffi's game in the early 90s was that her backhand had changed. In the late 80s, she had a very hard and vicious slice backhand that had a lot of block on it. In 1990 she broke her thumb in a skiing accident and when she came back during the claycourt season that year, she was hitting her backhand much closer to her body than before, making it go from a hard and heavy slice to a short and choppy one. She was also hitting it crosscourt all the time which made it predictable and both her down the line slice backhand and topspin backhand started to disappear as well.
Many of Graf's die-hard fans like to say it was solely because of the blackmail scandal in 1990 that she lost her dominance but it was much more than that. Yes, some of it was her problems like her groundstrokes changing after her thumb injury, some health issues, her hanging onto a very old and archaid Dunlop racket for years. But it was also because of the awesome rise of Seles (many Graf fans never acknowledge this which is ridiculous) and the improvement of Sabatini. I actually think Graf's toughest test in the early 90s was Sabatini because Sabatini's junkballing and chip & charge strategies exploited the issues with Steffi's game like her impatience and her unwillingness to move forward after hitting good shots. And the few times that Steffi would come into net in an effort to take the net away from Sabatini, Sabatini was really able to exploit that Steffi was very poor at reading the ball when at net.
The stabbing was terrible on many levels because not only would Seles have won more majors (people who say otherwise are completely deluded) but it robbed tennis of a potentially amazing rivalry. Because Steffi was showing signs of getting back into form during the years between 1993 and 1995. Graf looked much livelier in 1993 than in 91 and 92. And even more so in 1994 when she finally switched to a new Wilson tennis racket. But it was in 1995 that she at long last fixed the extension issue with her backhand. She also brought her down the line slice backhand back, she was hitting her second serve with a lot more topspin on it, and she was taking her forehand earlier than before. Unfortunately, Seles was out of action and Sabatini was sputtering badly. It's a real shame because this potential rivalry that didn't quite happen could've lifted tennis to amazing heights.
Dominic Thiem
for both Naomi Osaka and Kei Nishikori reaching the semifinals - they are the first Japanese players to reach the final four at the same Grand Slam. 
