The second set was a tighter affair with the Norwegian Ruud breaking in the fourth game, but at 4-3 down the Austrian toughed it out and brought the set back on serve.
It went to a tie-break and it was Thiem who emphatically sealed the match after clawing it back from 4-1 down with six straight points.
Rublev opens with victory over Struff
Andrey Rublev fights back to beat Jan-Lennard Struff at Thiem's 7
In the tournament's opening match, Andrey Rublev beat German Jan-Lennard Struff 3-6, 6-4,15-13.
Rublev was a participant in the Adria Tour and tested positive for Covid-19. But on Monday he confirmed he was cleared to play in the tournament after completing his self-quarantine period and testing negative for Covid-19 three times.
In the eighth game of a tight opening set, the Struff capitalised on his third break point to go 5-3 up before serving it out.
Rublev fought back in the second set, taking it on a tie-break at 5-4 up before emerging victorious in a tie-break decider with his fourth match point.
Berrettini, Bautista-Agut claim wins
Matteo Berrettini, who is set to play in the Ultimate Tennis Showdown semi-finals on Sunday, battled past Dennis Novak 7-6 (8-6), 6-3.
Novak saved four break points in a long game at 4-4 in the opening set but then couldn't convert three set-point chances in the next game.
Berrettini took his second set point in the tie-break and then secured an early break in the second set to put him on his way to victory.
Roberto Bautista-Agut prevailed 6-4, 6-3 in a top-20 battle against Karen Khachanov.
The Spaniard broke once in each set to secure victory.