Stan Wawrinka and Tomas Berdych were challenged before booking their spots in the Rogers Cup third round.
Wawrinka was tested by Russian Mikhail Youzhny, but came through a straight-sets winner in Toronto on Tuesday.
Fifth-seeded Czech Berdych was pushed to three sets before edging past Croatian Borna Coric at the ATP 1000 event.
The in-form Gael Monfils continued his good run, while two seeds exited.
WAWRINKA, BERDYCH CLAIM WINS
Wawrinka, the second seed, had lost three of his five previous meetings with Youzhny, but fought his way to a 7-6 (7-3) 7-6 (10-8) win.
Stan and the backhand have match point. pic.twitter.com/WfskTXu6wr-- Rogers Cup Toronto (@rogerscup) July 27, 2016
Berdych got past Coric with a 4-6 6-1 6-4 victory to make it three wins in as many matches against the teenager.
Fresh from winning the Citi Open, Monfils kept his good form going with a 6-3 6-3 win over Joao Sousa.
His fellow Frenchman, Lucas Pouille, edged to a victory of his own, beating qualifier Emilio Gomez 6-7 (8-10) 6-4 7-5.
PAIRE, JOHNSON EXIT
Only two seeds fell on Tuesday as Benoit Paire and Steve Johnson suffered straightforward losses.
Paire went down to Czech qualifier and veteran Radek Stepanek 6-1 6-3, while Johnson lost to Fabio Fognini 6-4 6-4.
KARLOVIC SERVES WAY PAST FRITZ
The 37-year-old Ivo Karlovic sent down 29 aces in a 6-7 (9-11) 7-6 (7-5) 6-2 win over Taylor Fritz, 18.
POSPISIL READY FOR MONFILS
Vasek Pospisil overcame Jeremy Chardy to set up a meeting with Monfils.
World number one and three-time champion Novak Djokovic begins his campaign against Gilles Muller. Kei Nishikori is also in action, taking on Dennis Novikov, while Milos Raonic faces Lu Yen-hsun.