Most likely. Or, it could just be he's reverting back to his real level. But the long break at 3 all in the first set is ominous. With that said, Haase deserves credit here, he's a better player overall than Matosevic with a longer history of playing good tennis.
The break was at 3 all (40-15) Matosevic serving. Before that moment Matosevic had won 10/10 points on serve, and shortly after they've restarted, Matosevic lost the set, winning only 1 point of the remaining 6 after 1st serve. Also, on bet365 I saw they even wrote there's an injury break happening.