Seles was a terrible match up for Graf and could really attack her backhand. Seles was also mentally tough and was not afraid of Graf and got under her skin in big matches. In fact, she relished competing with her, thrived on it. Graf had not really had anyone challenging her like that once she started dominating. Seles started beating her in big matches in 1990 and it wore on Graf quickly. She lost for the first time before a final at slams since USO '86 at Wimbledon '90 to Garrison. She lost the US Open final to Sabatini. At the 91 AO she lost to Novotna in the QF. Sanchez beat her in the RG SF 0 and 2. She certainly slumped and it did not help that Seles knocked her confidence. The tennis world was no longer Graf's, and it really shook her. All those losses in slams were to great players on those surfaces, but the old Graf would have edged them somehow or other, through belief more than anything. Seles took that belief from her and I imagine Graf was ill prepared for it to happen quite so soon. But Seles changed the game and when players like her come along, nothing is the same afterwards.
At Wimbledon '91, Graf got things back on track, but only just, vs Sabatini, who she was also struggling against after handling early in her career. Navratilova beat her at the USO'91 for the first time in 4 years. Seles would have been in the final. It's hard not to think that that might have preoccupied Graf a little.
In 92, Graf started to get to grips with the new order of things. She played a brilliant RG and an excellent Wimbledon. She still struggled, though, at the USO losing to Sanchez. There was also a loss to Capriati in the Olympics final.
This 90-93 Graf was a very different Graf to the dominant one. A much patchier one, still capable of huge highs, but not the mentally toughest player on tour anymore and prone to more lows than before. There was a lot going on off court in her personal life, but I am not accepting that as a reason for her dip in form. Graf was an absolute professional. She may have wanted to think she was distracted by off-court drama, but, imo, she was really rattled by what was going on in the tennis world and with Seles taking over at the top and beating her in big matches. It must have been very hard for Graf to imagine getting back on top the way Seles was so confident and simply, in big matches, mentally tougher.
Graf, played a great AO '93 final versus Seles, but that third set was ominous considering how things were going in slams for Graf overall, and she was certainly still able to compete with her, and had a fair few wins even in Seles' dominant period, but that was two slams in a year that Seles had proven to be the grittier and better player. The head to head was not lopsided, but Seles had a 3-1 lead in slam finals, and it's those matches that build or break the confidence of the likes of Graf and Seles. They were breaking Graf and building Seles.
I am not sure Graf would have gotten back to her good old days had Seles not been stabbed. I think she would have kept winning Wimbledon (94, 95, 96) and giving Seles big battles, maybe sneaking in another slam or two here or there (USO most likely). I think she'd have retired in '97 once the injuries kicked in, having had a great career and winning 13/14 slams and going down as an ATG but not a GOAT contender, leaving Seles to deal with Hingis and I think Hingis would have usurped Seles before the Williams' came along. Graf might have ended up sixth best after S. Williams, Navratilova, Evert, Seles and Court.
Seles getting stabbed gave Graf a new lease of life, but I am not convinced she would have gotten into her 95/96 shape with such confidence had she continued to suffer slam final defeats to Seles at 93 'RG and AO '94, etc.
So, yes, she was slumping, but it was Seles who got her into that slump, and Seles who got her out of it, on those occasions Graf got herself together in slams and Seles bought out the best in her, or the best of Graf's second best, and then, inadvertently and so sadly, it was Seles who helped Graf out of that slump for a much longer period.