ROTTERDAM, Netherlands (AP) -- Defending champion Martin Klizan of Slovakia beat Fernando Verdasco of Spain 6-4, 0-6, 6-1 to reach the second round of the World Tennis Tournament on Tuesday.
In an erratic contest, both players dropped serve four times. The unseeded Klizan, who had lost their two previous meetings, next plays Germany's Philipp Kohlschreiber.
Kohlschreiber advanced after beating eighth-seeded Lucas Pouille 7-5, 6-2, breaking the Frenchman's serve four times. The 22-year-old Pouille, ranked 17th and considered a future top 10 player, has been hampered by a right foot injury in the past month.
Sixth-seeded Jo-Wilfried Tsonga of France dropped serve three times in a 6-4, 7-6 (2) win against 18-year-old Stefanos Tsitsipas, a Greek player making his ATP main draw debut.
"It was difficult, as I did not know him at all," Tsonga said. "That's unusual on the Tour. I didn't know where to play at the start or what to expect."
Tsonga, ranked 14th, next plays Luxembourg's Gilles Muller, who had 14 aces in a 6-3, 6-2 win against Tallon Griekspoor - a Dutchman ranked 319.
Also, Gilles Simon beat last year's semifinalist Nicolas Mahut 3-6, 6-3, 6-4 in an all-French match.
Later, top-seeded Marin Cilic of Croatia was facing Frenchman Benoit Paire and Croatian Borna Coric was taking on Russian Karen Khachanov.