In the entire Djokovic-Nadal rivalry there have been a few instances, including today in Monte Carlo
, in which a player had higher success on second serve than on first serve.
Djokovic in his win in '07 in Miami - 64% on 1st serve, 75% on 2nd.
Djokovic in his win in '08 in Cincinnati - 71% on 1st, 76% on 2nd.
In their RG match in '08, which Nadal won, this happened to both men. Nadal was 66% on 1st and 70% on 2nd. Djokovic was 57% on 1st and 58% on 2nd.
Djokovic in his win in '11 in Miami - 66% on 1st, 70% on 2nd.
Djokovic in his loss today in Monte Carlo - 40% on 1st, 56% on 2nd. This one really stands out because the success on 1st serve is so low.
Just a note, I don't have stats for two of their matches: at the Olympics in '08 and in Davis Cup in '09.