Second-seeded Marin Cilic of Croatia reached the World Tennis Tournament quarter-finals with a 7-6 (7-2) 7-6 (9-7) second-round win against Gilles Muller of Luxembourg.
Neither player faced a break-point in the first set, but Muller saved five in the second.
Cilic had two match points at 6-4 in the tiebreak, but then saved a set point at 7-6 down as Muller fought back strongly.
The 2014 US Open champion next plays Philipp Kohlschreiber, who won 6-4 6-2 against qualifier Ivan Dodig of Croatia.
The German proved clinical on Dodig's serve, converting four of five break-point chances, and solid on his own - winning 75 per cent of points on second serve.
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Later, fifth-seeded Gael Monfils of France played 19-year-old Borna Coric of Croatia.
There were also remaining first-round matches.
