While the first half of the day's running took place in wet conditions, the track dried out sufficiently for drivers to switch to slick tyres in the afternoon session.
Two-time championship runner-up Nico Muller was the first driver to break the 1m40s barrier during the day, setting a time of 1m39.899s to go quickest in the Team Rosberg Audi.
This was quickly usurped by factory Porsche driver Laurens Vanthoor in the SSR Performance 911 GT3 R, before HRT Mercedes newcomer Luca Stolz took to the top with a 1m39.307s.
Stolz managed to gain more time on his next flying lap, but reigning GT Masters champion Ricardo Feller put Audi back at the head of the timesheets with a 1m39.270s.
There was more time to be gained in the remainder of the session, with DTM rookie Zug taking advantage of improving conditions to post a lap of 1m38.344s and end the day the quickest of all for the Attempto Audi team.
Three-time DTM champion Rast was competitive straight from the get-go on his return to the series, ending up 0.338s behind Zug in the lead Abt Sportsline entry.
The two Audi drivers finished well clear of the rest of the field, with their nearest challenger Lucas Auer ending up a further half a second adrift in the Winward Mercedes.
Fourth was Muller after improving to 1m39.088s late in the session, with two-time DTM champion Marco Wittmann (Walkenhorst) putting the new BMW M4 fifth on the leaderboard ahead of Stolz and Feller.
The top 10 was rounded out by Mucke Mercedes' Maximilian Buhk, Schubert BMW driver Philipp Eng and defending champion Maximilian Gotz for HRT Mercedes, the latter ending up just over a second off the pace with a time of 1m39.681s.
Factory Lamborghini driver Mirko Bortolotti set the pace in the wet morning session with a time of 1m48.866s, but ended up 13th on the overall timesheets, two places behind GRT teammate Rolf Ineichen.
Vanthoor ended up 14th on the leaderboard, two places ahead of teammate Dennis Olsen, while Thomas Preining was the last of the Porsche drivers in the Team Bernhard 75-operated 911 GT3 R in 24th position.
Red Bull duo Nick Cassidy and Felipe Fraga were classified 17th and 18th respectively, separated by just 0.061s in the two AF Corse-run Ferraris.
Sebastien Loeb took over the AlphaTauri-liveried car from Cassidy during the final hour as he began preparations for his DTM debut at Portimao. However, his best time remains unclear as the transponder on his car wasn't updated to reflect the driver change.
A total of 28 out of 29 drivers were present on Tuesday, with T3 Motorsport driver Esmee Hawkey not travelling to Hockenheim after reportedly testing positive for COVID-19.
Her teammate and Aston Martin factory driver Nicki Thiim showed impressive pace in wet conditions, but was only 23rd quickest in the dry as he continued to acclimatise himself with the Lamborghini Huracan GT3.
Hockenheim DTM - Day 1 results:
| P | Nr | Driver | Car | Time | Gap |
|---|---|---|---|---|---|
| 1 | 66 | M. ZUG | AUDI | 1:38.344 | - |
| 2 | 33 | R. RAST | AUDI | 1:38.682 | 0.338 |
| 3 | 22 | L. AUER | MERCEDES-AMG | 1:39.082 | 0.738 |
| 4 | 51 | N. MÜLLER | AUDI | 1:39.088 | 0.744 |
| 5 | 11 | M. WITTMANN | BMW | 1:39.117 | 0.773 |
| 6 | 4 | L. STOLZ | MERCEDES-AMG | 1:39.213 | 0.869 |
| 7 | 7 | R. FELLER | AUDI | 1:39.279 | 0.935 |
| 8 | 18 | M. BUHK | MERCEDES-AMG | 1:39.529 | 1.185 |
| 9 | 25 | P. ENG | BMW | 1:39.574 | 1.230 |
| 10 | 1 | M. GÖTZ | MERCEDES-AMG | 1:39.681 | 1.337 |
| 11 | 19 | R. INEICHEN | LAMBORGHINI | 1:39.746 | 1.402 |
| 12 | 10 | E. MUTH | BMW | 1:39.761 | 1.417 |
| 13 | 63 | M. BORTOLOTTI | LAMBORGHINI | 1:39.795 | 1.451 |
| 14 | 92 | L. VANTHOOR | PORSCHE | 1:39.973 | 1.629 |
| 15 | 85 | C. SCHMID | LAMBORGHINI | 1:39.985 | 1.641 |
| 16 | 94 | D. OLSEN | PORSCHE | 1:40.008 | 1.664 |
| 17 | 37 | N. CASSIDY | FERRARI | 1:40.131 | 1.787 |
| 18 | 74 | F. FRAGA | FERRARI | 1:40.191 | 1.847 |
| 19 | 36 | A. MAINI | MERCEDES-AMG | 1:40.232 | 1.888 |
| 20 | 31 | S. VAN DER LINDE | BMW | 1:40.276 | 1.932 |
| 21 | 12 | D. GORE | AUDI | 1:40.405 | 2.061 |
| 22 | 27 | D. SCHUMACHER | MERCEDES-AMG | 1:40.579 | 2.235 |
| 23 | 95 | N. THIIM | LAMBORGHINI | 1:40.632 | 2.288 |
| 24 | 24 | T. PREINING | PORSCHE | 1:40.689 | 2.345 |
| 25 | 3 | K. VAN DER LINDE | AUDI | 1:40.691 | 2.347 |
| 26 | 99 | M. GRENIER | MERCEDES-AMG | 1:40.702 | 2.358 |
| 27 | 88 | M. ENGEL | MERCEDES-AMG | 1:40.956 | 2.612 |
| 28 | 6 | A. DELEDDA | LAMBORGHINI | 1:41.780 | 3.436 |
