MONTREAL, Canada (AFP) — Defending champion Ben Shelton kept a clean slate at the ATP Montreal Masters on Sunday, holding off a challenge from Joao Fonseca to power 6-3, 7-6 (7/3) into the quarterfinals.

World No. 10 Shelton has not dropped a set in reaching the Last Eight.

The 23-year-old American left-hander will play for a semi-final berth against 13th-seeded Czech Jakub Mensik, who beat Botic Van De Zandschulp 6-4, 7-5, claiming victory on a winning lob.

Van De Zandschulp served for the second set but was broken, with Mensik seizing the momentum as he won 12 of the final 14 points.

Brazilian teenager Fonseca, billed as one of the faces for the future of the sport, might have compromised his own chances when he landed badly on his ankle after missing a smash while leading 3-1, 40-30 in the second set.

The youngster was plagued by 32 unforced errors as his opponent mastered an annoying evening wind to earn the victory.

"It was tough today, not easy conditions," Shelton said. "With the wind I struggled to find footing on a lot of balls.

"It all came down to grit. The tennis was not clean. Beating a high-quality player like that is huge for me."

Shelton secured his ninth straight victory in Canada on the second of four match points as Fonseca hit the net with a return after leading in both sets.

Daniel Merida smothered Tallon Griekspoor 6-3, 6-1 as the Spaniard continued his rapid adjustment to hardcourt tennis in Canada.

The world No. 63 joins compatriot Rafael Jodar in the Canada quarter-finals and will break through to a career-best spot inside the top 50 as a result of his fourth-round victory.

Prior to making his Montreal debut, clay-courter Merida had never won a tour-level match on hardcourt.

He will try to extend his streak of cement success to five in a row when he plays a first Masters quarterfinal against US 12th seed Learner Tien, who wore down Argentine Thiago Tirante 6-4, 6-4.

Tien saved the first eight break points he faced in the second set in a battle of big hitters and secured victory in his fifth match point, with his opponent handing over the final game with four unforced errors.

"Today was humid, conditions are always tough," Tien said. "I'm happy with how the match worked out."

"This was one of those days where I was making good contact with the ball. I was seeing the ball well and returning well."

Against Griekspoor, Merida was deadly with his backhand, leaving the Dutchman flat-footed in the 69-minute triumph.

Spain's title-winner last month on clay in Umag has now claimed 16 victories this breakthrough season.

"This is a dream come true. I'm feeling really great," Merida said. "I'm happy with the level during all the week.

"I was playing great for the last year and the beginning of this year so I just started to believe more in myself."