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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
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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.
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.
DJOKOVIC FACES MULLER
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.