The discussions for a Mayweather-Pacquiao bout started in late 2009 and continued into 2010; they had settled on all elements of the agreement except for the drug-testing guidelines. Mayweather demanded random, Olympic-style blood and urine testing, while Pacquiao refused to accept the particular type of drug testing, leading to the collapse of the deal.
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Some say after Pacquiao's brutal beatdown of Cotto after getting heavier but keeping the same speed and power, Floyd was suspicious of his stamina.
This lead to their highly anticipated fight being postponed by 5 years to 2015, leaving some people to say Floyd waited until Manny slowed down.

Manny Pacquiao's blood test refusal puts Floyd Mayweather Jr fight at risk
Manny Pacquiao's refusal to take a blood test in the 30 days before a fight has delayed an agreement with Floyd Mayweather Jr

Shame on Floyd Mayweather, Manny Pacquiao for allowing drug testing dispute to kill fight
Ten days separated Mayweather and Pacquiao from March 13 bout.
Some say after Pacquiao's brutal beatdown of Cotto after getting heavier but keeping the same speed and power, Floyd was suspicious of his stamina.
This lead to their highly anticipated fight being postponed by 5 years to 2015, leaving some people to say Floyd waited until Manny slowed down.