Andy Murray survived a scare in his first-round match at the Vienna Open, while Kei Nishkori and Juan Martin del Potro triumphed comfortably at the Swiss Indoors.
Top seed Murray was forced to three sets by Slovakian Martin Klizan, but displayed his class to see out a 6-3 6-7 (5-7) 6-0 victory.
The Briton, chasing down Novak Djokovic at the top of the rankings, let a 5-3 lead in a second-set tie-break slip, but won the following six games in succession to set up a meeting with Gilles Simon in the second round.
Should Murray triumph he would face John Isner in the quarter-finals after the American downed Feliciano Lopez 7-6 (7-1) 3-6 7-6 (7-2).
There were also wins for eighth seed Ivo Karlovic, Philipp Kohlschreiber, Albert Ramos-Vinolas and Karen Khachanov.
In Basle, Del Potro cruised to a 6-3 6-4 victory over qualifier Robin Haase, with fifth seed David Goffin – in the hunt for a place at the ATP World Tour Finals – awaiting in the second round.
After that the Argentine, who won his first ATP title in 33 months at the Stockholm Open last weekend, faces a potential showdown Nishikori after the Japanese ousted Paolo Lorenzi 7-6 (7-3) 6-2.
Pablo Carreno Busta set up a second-round match against fourth seed Marin Cilic by defeating fellow Spaniard Fernando Verdasco 7-5 6-3, while seventh seed Richard Gasquet retired trailing 6-2 2-1 to Guido Pella.
Jack Sock, Mischa Zverev and Federico Delbonis also registered victories.