Merckx and Sercu formed a legendary partnership on the track, winning 15 six-day events together.
Sercu's death was announced by his son, Christophe, on Friday, who explained that his father's health "had been unstable for several years and had deteriorated sharply in recent weeks". Sercu died at the age of 74.
"He was a friend I had known since I was 16 or 17. This is a big loss for me," Merckx told Belgian broadcaster Sporza.
"Our partnership goes back a long way - from the 1960's on the Brussels velodrome. We were rivals at first, then we became teammates. Together, we won 15 six-day races, including the Gent Six in 1965. They are wonderful memories.
"He was a fantastic guy, a good man. He was honest, did not beat around the bush, and always looked for compromise. I appreciated that in him and that is why I rode with him for so long."
Sercu racked up no fewer than 1206 victories during his career, most of which came on the track. He won Olympic Gold in Tokyo in 1964, and won two world titles as well as 11 victories in the Gent Six.
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Merckx, widely considered the greatest cyclist of all time, insisted Sercu was the superior rider on the track, but also pointed out the fact he had won 168 races on the road, including 13 stages of the Giro d'Italia and six at the Tour de France.
"Patrick had the speed and I had the endurance. We were a good pair. But he was definitely a better rider than me on the track," Merckx said.
"He could do much more than just ride on the boards. He won the green jersey at the Tour de France [in 1974]. He broke through in a period where there were many great champions in Belgium. With that, his career on the road might be somewhat underexposed."
Tributes have flooded in from the cycling community since news of Sercu's death.
Iljo Keisse, a decorated six-day rider who competes on the road for Decueninck-QuickStep, said: "The news hit my face like a blow. I knew Patrick very well. Ever since childhood, actually. He was my manager for a while and then my race leader in numerous six-day races.
"I certainly looked up to him as a little rider. Take a look at his palmarès. That is immense. He won a pack of six days, a few Belgian and world titles, in addition to an Olympic title. Phenomenal. It hurts to hear that he is no longer here. I undoubtedly owe part of my career to him."
