Lewis finished with 11 points while Jonathan Holmes had 10 for Texas.
"We were very close out there," Lewis said. "We keep losing these close games by possessions. It hurts us a lot. But we got to keep going."
The Jayhawks played listless at the start.
McLemore showed no hint of an injury after spraining his ankle in Monday night's win over Baylor, but the Big 12's second-leading scorer took just three shots in the first half while his teammates missed shot after shot.
Defense hasn't been the problem for the Longhorns during this forgettable season — their offense takes the blame there. That showed while holding Kansas to a season-low 26 points in the first half on 28 percent shooting, which was easiest to notice as the Jayhawks went 1 for 7 from behind the arc.
That lone 3-pointer belonged to McLemore in the opening minutes to give Kansas the lead, something the Jayhawks wouldn't get back until the second half when the star freshman made another 3. And even that didn't last long: Texas answered with baskets on five straight possessions, stretching their lead to 11.
But then the Longhorns did what's doomed them all season.
They gave away another late lead.
Up by eight against UCLA in December. A 13-point lead against West Virginia in the Big 12 home opener. Both went down as losses, and so did this one after Kansas shook off what Self called a "miserable" start to the second half.