World number one Carlos Alcaraz claimed a hard-fought win while Novak Djokovic also triumphed as the pair advanced at the Cincinnati Open.

World number two Djokovic maintained his unbeaten record against France's Gael Monfils, winning 6-3 6-2.

The top-ranked pair are preparing for the US Open, which begins on 28 August.

23-time Grand Slam champion Djokovic, a two time winner in Cincinnati, dominated the baseline rallies and made the most of a loose service game from his long-time rival to break to love at 5-3 in the first set.

After racing in to a 5-0 lead in the second he faltered on his first attempt to serve it out, but completed the win two games later in an hour and eight minutes

"[Monfils and I] been playing each other in different parts of the world, also in juniors," said Djokovic, who will next face home favourite Taylor Fritz. "Early on, I struggled with him physically.

"We haven't played for quite a few years. It's great to see him back playing at a high level."

Alcaraz had to deal with a 90-minute rain delay midway through the third set of his match before winning to set up a quarter-final meeting with Australian qualifier Max Purcell, who beat Stan Wawrinka of Switzerland 6-4 6-2.

"It was a really tough match," said Alcaraz, who will defend his US Open title later this month. "We were playing a really close match before the rain came. But I think I did pretty well [dealing with] the wait in the gym, warming up.

"I told everyone that I really wanted [to win] since I lost in Toronto. I think I'm getting better and better. I'm really happy to be in the quarter-finals here."

Germany's Alexander Zverev has lost to Daniil Medvedev at three Masters 1000 events this year but gained some revenge to claim his biggest win since undergoing ankle surgery last year.

The 26-year-old won the Hamburg Open last month and stepped up his preparations for the US Open, where he lost in his only previous Grand Slam final in 2020, by beating world number three Medvedev 6-4 5-7 6-4.

Zverev will face Adrian Mannarino in the quarter-finals. The Frenchman was 6-4 3-0 up against Mackenzie McDonald when the American was forced to retire.

World number four Stefanos Tsitsipas was upset 6-3 6-4 by Poland's Hubert Hurkacz, who will next face Alexei Popyrin after the Australian beat Finland's Emil Ruusuvuori 6-2 1-6 6-3.