The Spaniard was the eldest of the duo when they teamed up for Toro Rosso in 2015 before Verstappen was eventually called up to replace Daniil Kvyat at Red Bull part-way into the 2016 season.
When asked as to what was most difficult to deal with as Verstappen's teammate, Sainz struggled to select a single reason.
"Max is, was and has become one of the strongest, if not the strongest driver on the grid right now and he is very, very, very good," he said.
"He is extremely competitive. I admired him, I enjoyed being his team-mate, I actually learned a lot. It has made me a better driver for it clearly. Just good things about him on the driving side and he's a sociable guy."
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