Altmaier out-aced 34-year-old Querrey, who has served the fifth-most aces of all time, by seven to two. The American’s serve was not at its usual potency, as he also served eight double faults and only made 42 per cent of first serves.
Querrey, a 10-time ATP Tour champion and former World No. 11, is now 0-7 in his past seven matches.
The 23-year-old German went on back-to-back semi-final runs earlier this year in Umag and Kitzbuehel, also reaching the round of 16 at Roland Garros in 2020 as a qualifier.
Up next for Altmaier is former World No. 3 Grigor Dimitrov, a semi-finalist in San Diego last week.
The victory is Popyrin’s first over Kecmanovic, with the 22-year-old Serb winning their first two encounters in straight sets.
Popyrin, who won his first ATP Tour title earlier this year in Singapore, is recapturing his form towards the end of the season. The Australian has now won six of his past eight matches.
Kecmanovic’s YTD record now stands at 14-23, and he is 1-7 in ATP Masters 1000 events in 2021. The current World No. 62 won his lone ATP Tour title last season in Kitzbuehel.
Carlos Taberner won an all-Spanish affair against Jaume Munar 6-3, 6-3, while Japan’s Taro Daniel defeated German Philipp Kohlschreiber 6-2, 6-4. Taberner will next face fourth-seeded Russian Andrey Rublev in the second round, while Daniel will take on American Reilly Opelka, the 16th seed.