Clay City built up a double-figure lead in the second half and held on for a 74-71 victory.
The loss was the third in as many games for Union, who slipped to 0-2 in the SWIAC. Clay City evened its record at 1-1 overall, 1-0 in league play.
"They were able to come back in this game. I think we were down eight (34-26) at the half and didn't play very well in the third quarter. But the boys really responded and battled right to the end and gave themselves a chance."
Robertson finished with career highs of 18 points, 12 rebounds and four blocks. After playing mostly junior varsity last year, this was just his third varsity start.
He took a pass in the lane from Ethan Fettig and finished in traffic as Union cut the Eel advantage to 67-63 with 46 seconds remaining.
"This was the best game of Austin's short career," said Langley. "He has worked really hard to improve his game and you can tell he has gotten better.
"He is just a sophomore, so I am very encouraged by his progress and the fact he has almost three full seasons to play."
Sterling Langley, who finished with 12 points, made three of his four 3-pointers in the fourth quarter. He buried one from the right wing with 5.5 seconds remaining as the Bulldogs climbed within 70-69.
Clay City's Logan Cessinger hit 6 of 7 free throws over the final 70 seconds to help the Eels seal the victory. He made 13 of 16 free throws and finished with a game-high 30 points.
"He really hurt us with his quickness and ability to get to the basket and the foul line," Langley said of Cessinger. "We have good size, but need to work on our footwork and quickness."
Evan Rogers made five 3-pointers and added 18 points for the Eels.
After Zac Birdsong missed a potential game-tying 3-pointer with 5.5 seconds to play, Steven Phillips then hit two foul shots for the Eels and then Robertson scored an unguarded basket as time expired.
Fettig tied Robertson for team-high honors with 18 points. Birdsong added 10 as four Bulldogs scored in double digits.
"The was a good step in the right direction, but we have to forget about this by Saturday morning and get ready for our next opponent," said Langley.
Clay City also won the junior varsity game 49-32. Parker Lovelace had nine points for the Bulldogs, Brayden Talley eight, Melidian Maze seven, Trey Bedwell six and Dakota Chesterfield added two. Raylee Mesey, Skyler Mesey and Clay Figg also played.
The Bulldogs will turn around and host Eminence at 6 p.m. Saturday (tonight).
CLAY CITY (74) -- Paiden Myers 0 2-2 2, Joshua Arcenai 0 0-0 0, Easton Booe 3 0-0 6, Steven Phillips 3 2-2 9, Logan Cesinger 8 13-16 30, Evan Rogers 5 3-8 18, Trevor Singer 1 1-2 4, Kurtis Yocum 0 1-2 1, Parker Ames 0 0-0 0, Josh Miller 2 0-0 4. Totals 22 22-32 74.
UNION (71) -- Ethan Fettig 7 4-7 18, Ian Hamburg 4 2-2 9, Austin Robertson 8 2-2 18, Zac Birdsong 4 0-0 10, Sterling Langley 4 0-2 12, Trey Bedwell 1 2-3 4, Brayden Talley 0 0-0 0. Total 28 10-16 71.
Clay City 16 18 18 22 -- 74
Union 13 13 17 28 -- 71
3-pointers -- Clay City 8 (Rogers 5, Singer, L. Cessinger, Phillis), Union 7 (Langley 4, Birdsong 2, Hamburg). Fouled out -- Bedwell, Hamburg.