Carlos Alcaraz has made it easier on himself second time round at the Madrid Open, reaching the round of 16 with a straight-sets victory over Grigor Dimitrov.

After struggling to a three-set win in his opener against unseeded Emil Ruusuvuori, Alcaraz improved to get past Dimitrov 6-2 7-5 and stay on track to defend his title on home soil.

Alcaraz was still not at his best but sealed the victory by converting four of his six break opportunities against the 26th-seeded Bulgarian.

The world No.2 Spaniard - top seed in Madrid - was broken only once, in the second set. He finished with 16 winners and four unforced errors.

"We didn't do some things well in that first match and it was clear that we had to get back to doing what we know, which is to go on attack," Alcaraz said.

It was the 17th straight victory on home soil for the 19-year-old, who earlier this month won the Barcelona Open for a second consecutive year.

He next faces 13th seed Alexander Zverev in a rematch of last year's final.

"I have great memories from last year in the final but I have to be really focused in that match," Alcaraz said.

"I know that Sascha (Zverev) is a really good player, a really aggressive one with good serves. I have to show all my skills."

Two-time Madrid Open champion Zverev set up the last-16 clash by cruising past qualifier Hugo Grenier 6-1 6-0 in less than an hour.

"Today I felt great," Zverev said.

"Maybe Hugo was a bit nervous, first time in the third round at a Masters and that is big for any player.

"I am happy I kept my composure and that I am through."

Zverev, who had 26 winners and broke Grenier's serve six times, won his Madrid titles in 2018 and 2021.

He is 21-3 in the tournament, having reached the quarter-finals in all five of his previous appearances.

Fifth seed Andrey Rublev improved to 10-1 on clay this season with a 6-2 7-5 win over Yoshihito Nishioka.

Rublev, who reached the quarter-finals in Madrid last year, won the Monte Carlo Masters two weeks ago.

He next faces Karen Khachanov, the 10th-seeded player who defeated home favourite Roberto Bautista Agut 7-5 4-6 6-3 in a match that lasted almost three hours.

Australia's last-man standing Alex de Minaur is scheduled to take to the court against Russia's Aslan Karatsev on Monday evening (AEST).