Gilles Simon sprung a surprise on Denis Shapovalov in Cologne, where Diego Schwartzman made serene progress to round three.
Top seed Alexander Zverev has won 11 of his last 12 hard-court matches, but it was not all plain sailing against John Millman on Wednesday.
Andrey Rublev and Denis Shapovalov will fight it out for a place in the final of the St. Petersburg Open on Saturday.
Andy Murray entered the ATP 250 tournament in Cologne to build up match practice, but he tumbled out in the first round on Tuesday.
Fabio Fognini paid for a sloppy performance as he crashed out of the Internazionali d'Italia. Andrey Rublev and Milos Raonic also departed.
Lorenzo Musetti was three when Stan Wawrinka made his Internazionali d'Italia debut but the home hope downed the three-time grand slam king.
Andy Murray's US Open campaign ended with a second-round loss to Felix Auger-Aliassime.
Last year's US Open runner-up, Daniil Medvedev was in fine form in New York.
Novak Djokovic revelled in his triumphant display at the Western & Southern Open on Saturday.
Just two days before the US Open gets under way, Novak Djokovic showed why he is the favourite to win an 18th major title.