What happened after the first round? Or let alone, the rest of the fight?
It was a split decision that many thought BJ Penn (should have) won.
The second fight - BJ in decline, GSP in ascendency. Better gameplan by GSP with the standup grappling/dirty boxing version of lay and pray, followed by more dry humping. GSP domination.
Two things have hurt BJ Penn in his career: poor dedication to cardio training and no true weight class for him. He's a bit big to be a lightweight and too small to be a welterweight. Both were exposed by GSP and Nick Diaz.
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I see Nick Diaz giving GSP the toughest bout of his career. And his two areas of advantage - boxing and BJJ/subs - are huge factors. Plus we know Nick Diaz is tough and has no quit in him - GSP will pretty much have to knock him out, choke him out or snap a limb to stop Diaz. He won't break him. And we haven't seen GSP stop anyone in their prime in a LONG time.
Diaz's relentless punching will give GSP fits - but his lack of one-punch power is one area GSP's weakness where he'll not have to be overly concerned. Of course the same was said of Serra, pre-upset. And Nick's BJJ from the bottom (I 'think' Georges will take him down almost at will, but we haven't seen Diaz vs. a good wrestler in a long time, so who exactly knows?) will threaten St. Pierre and make the fight even more dry-hump-ilicious or will end badly (read: sub) for GSP if he really is p.o.'d and gets over-eager.
I just don't see GSP taking any chances and see this fight being similar to the Jake Shields one, with Diaz being more competitive. GSP 48-47.