Play in the Round of 64 at the Monte Carlo Masters opened on Monday after two days of qualifiers, though it was severely delayed due to heavy rain. 

After an hours-long rain delay, play resumed but didn't get too far in. When the tournament moved to a second rain delay, five matches were suspended and four had yet to take the court.

Only four were completed before play was called around 7:30 p.m. Monte Carlo time.  

Monday Results

Tommy Paul def. Pedro Martinez 4-1 (walkover)

Aslan Karatsev def. Lorenzo Musetti 6-3, 6-4

John Millman def. Ugo Humbert 6-3, 6-3

Alejandro Davidovich Fokina def. Alex de Minaur 6-4, 7-6 (7)

Updated draw available from ATP Tour. 

Recap

The only American player to take the court on Monday, Tommy Paul, took the win over Pedro Martinez, who bowed out after going down 4-1. 

Paul awaits the winner of Tuesday's match between the United States' Taylor Fritz and Spain's Roberto Bautista Agut, the No. 9 seed.

While the United States was quiet on Monday, Australia was well-represented, with the nation's top two players battling it out in separate matches.

The top-ranked player in Australia, Alex de Minaur, lost his fourth consecutive match when he dropped a 4-2 lead in the first set, and finished the day having lost six of eight break points to send Spain's Alejandro Davidovich Fokina into a matchup with No. 8 Matteo Berrettini. 

But Australia's No. 2, John Millman, had better luck. He won four of five break points throughout the day, limiting France's Ugo Humbert to 48 percent success on second serves and winning as many as 13 points in a row. 

Millman will play the winner of the match between Felix Auger-Aliassime and No. 16 Cristian Garin. 

Aslan Karatsev had a strong start to his time in Monaco. In his first match of the Monte Carlo Masters on Monday, he shut down Italy's Lorenzo Musetti, 6-3, 6-4 to earn his first ATP Tour victory on a clay court.

The Russian is having a year of firsts, apparently. After reaching the semifinals in the Australian Open as a qualifier—his first Grand Slam semifinal—he earned his first ATP Tour victory with a win at the Dubai Duty Free Tennis Championships. 

Musetti, who is a resident of Monte Carlo and the youngest player in the ATP Tour Top 100 at 19, put up a good fight against Karatsev, limiting the 27-year-old to 60 percent success on his second serve after allowing an 82 percent victory to the Russian on his first serve, but he couldn't get a streak going. He never won more than one game in a row and allowed Karatsev to save all four break points. 

Karatsev will face No. 4 Stefano Tsitsipas in the Round of 32.