The 23-year-old was torn between celebrating being the second player behind India's run machine Virat Kohli to achieve the 1,000 milestone and ruing Sri Lanka's precarious position with a day remaining in the series deciding Test.
Sri Lanka resumed the day at 24 for two with Mendis and skipper Dinesh Chandimal steeled to bat through all three sessions to give the tourists a chance to draw the match and series.
Instead, after facing 147 deliveries he was out to a loose shot midway through the middle session and at stumps on day four Sri Lanka were 231 for six with their 660-run target out of sight.
Of reaching 1,000 runs, Mendis said he was "really glad I was able to do it at 23, and I had challenged myself to get there in this last game."
When he was out for 67, caught at extra cover off Neil Wagner, Mendis had 1,023 runs from 23 innings at 46.50. Kohli has amassed 1,322 from 24 innings at 55.08.
"I'm really happy. I did have that target in mind," he said.
"In the first few matches of the year I didn't have that in my head, but later on, after I'd played a few games this year, and scored a few runs, then I became pretty intent on getting to 1,000 runs."
However, his dismissal when Sri Lanka needed a long partnership took the gloss off the day.
"That's a bit disappointing," he said.
"The plan was to bat as many balls as possible, as had been the case in Wellington. I tried to make the ball older and softer. If runs came, then that's a bonus. So we took it hour by hour and tried to break the work up that way.
"I was able to bat a session and a half, but if I'd been able to support Chandi a little bit better, we might have been able to finish today without a wicket as well."
Wagner lured Mendis into the dismissal shot with a series of short balls that forced him on to the back foot and then pitching one up the brought the lofted drive.
"I don't think that was a great ball to drive. After looking at the replays, that's probably what I take from it. I was trying to hit through extra cover. The fielder was close, and I hit it too far in the air.
"It's one that I really could have left alone, but I made the wrong decision. Hopefully, I can learn from it."
First Published: December 29, 2018, 2:40 PM IST