He was meant to be playing compatriot Steve Johnson today, in a highly anticipated match up, but won't be taking the court due to fatigue.
The American only arrived in New Zealand in the early hours of Tuesday morning.
Pella and Harrison will be replaced by two 'lucky losers' from the qualifying, with American youngster Liam Caruana and Japanese Taro Daniel coming into the draw.
Their exits come on top of the departures of Andrey Rublev and Kyle Edmund on Monday.
Highly rated Russian Rublev, who was the eighth seed for the tournament, pulled the pin yesterday citing an arm injury, after reaching the final in Doha last week.
Edmund has an ankle sprain.
It means that there will be a total of four lucky losers on court today, which gives an underwhelming feel to the second day of the tournament.
It's an issue that continues to dog the ATP tour. There is no doubt that some of the injuries are genuine, but it's also not unknown for players to pull out of a tournament the week after having a good run.
Players are still entitled to their first round prizemoney - around US$5,075 ($7,062) in a rule changed recently last year by the ATP.
That rule was altered to prevent the practice of players going on court with an injury, knowing they won't be able to finish the match, but just wanting to claim their prizemoney.
There is also an incentive for players to turn up, even if they don't intend to play, as there is a US$4000 ($5568) fine if players enter a tournament and then don't show up.
