Alonso has scored three podiums in as many starts for his new team after joining from Alpine.
It stands as the best start to a season the squad had since its foundation as Jordan in 1991.
“It's fantastic,” Krack said of Alonso's early performances.
“If I was asked before, if someone tells you, ‘you have three podiums after the first races', I would not have believed it either.
“But it shows also what a champion we have there,” he added of the two-time world champion.
“It's just incredible, his consistency. If you look at all the sessions so far throughout the year, he has always been there.
“In every session; free practice 1, free practice 2, he's always up there and pushes at the maximum all the time.”
‘Happy days' at Aston Martin for Alonso
Alonso has stated it's “happy days” at Aston Martin for the moment, though warns of the challenge that lies ahead.
“But from now on, maybe we will see the level of the teams changing a little bit, race-by-race, depending on who brings an upgrade that is good enough,” he warned.
Formula 1 is in the midst of an early season break.
The cancellation of the Chinese Grand Prix has left a hole in this year's F1 calendar.
From Melbourne, the sport instead heads to Azerbaijan for Round 4 at the end of April.
That event is poised to see the real start of this year's development race with a number of teams set to bring updates to the Baku streets.
Aston Martin is not planning on changing its approach and will stick to its programme despite early success.
“It does not really change anything on our plans,” Krack said of the break before the next event.
“[China] was on the calendar, then it got cancelled, so we use it to do our work.
“But I'm sure all the other teams are doing this in the same.
“So again, [the] development race has started and it is independent of if you have a race now.
“Everybody will be flat out.”