"It was difficult to play shots as the ball got older. You have to wait for the ball to come on.
"
The opener, who went into Wednesday's match with just 17 runs to his name in his previous three innings, was expected to be replaced by veteran former skipper Marvan Atapattu.
But the 22-year-old left-hander made his 73 off 74 balls and was quick to pay tribute to captain Mahela Jayawardene and coach Tom Moody for sticking by him.
"The captain and the coach came to me and told me I was playing and that gave me a lot of confidence," said Tharanga who was his side's second highest scorer behind Jayawardene's blistering unbeaten 115.
"They believed in me as did everyone else in the team.
So I'm very happy but I was a little disappointed to get out."
Tharanga was eventually third man out with the total on 111 in the 26th over when he was bowled around his legs by spinner Daniel Vettori.