I have seen too many similarities between Nadal and Seles:
1)Nadal is the King of clay, winning 3 Roland Garros beating 2 times Federer,while Monica won also three French Opens beating there 2 times Steffi Graf.
2)Nadal is improving his game as Seles did in the first stage of her career she launched first as a clay courter,same as Rafa.
3)Nadal is mentally the strongest player on the circuit, Seles mental strength was very unquestionable too.
4)Seles was pushing Steffi on her throne everytime more and more from 89 until 91 when finally claim the top spot...Nadal is trying the same feat as we all know.
5)Rafa reached last year's Wimbledon final, Seles reached in 92, both were beated by Federer and Graf (their archi-rivals)
6)Rafa average of won finals is very high more than 93%, first stage Seles has also a very impressive percentage of wins over losses on finals.
7)Rafa has the same lovers/haters meanwhile everybody seems to want Federer always win, when Seles faced Graf everebody was on Steffi's side.
8)Everybody saw from the beginning Seles as a menace to Graf's number 1 spot, while Nadal is the Roger's.
9)Both were the most successful teenagers of their times.
10)Rafa's game is very incisive to the lines, Monica also loved kissing court lines.
11)Seles-Graf held the top 2 spots on world rankings for more than 3 yeas (think from mid 1900 to mid 1993)while Nadal-Federer are just past 2 years helding the 2 pole positions (I think from june 2005)
This is some simlarities that came to my mind quickly...if you can see also more to add I'd like you to share with us!