Rafael Nadal overcame a slight blip in the first set before beating Stanislas Wawrinka 7-5, 6-4 for his 40th straight win at Monte Carlo.
Nadal, who followed World No. 1 Novak Djokovic on to the court, broke Wawrinka immediately and then opened up a 3-1 lead.
The six-time French Open champion looked in control at 40-15 in the eighth game, but some uncustomary unforced errors gave Wawrinka a chance and he broke back for 4-4.
Wawrinka briefly led at 5-4 when he fought back again from 40-15 down and held with a strong second serve. Nadal won the next three games to win the set. He set up match point with a pass down the line and closed it out with an ace.
Tennis great Bjorn Borg watched Nadal's victory, then said on television that he considered the Spaniard the greatest player ever on clay.
"Let's wait (until) I finish my career," Nadal said. "My uncle always told me that when Bjorn goes on court, everybody feels that he's unbeatable. I don't know how to explain but I don't have this feeling."
The Swede won six French Opens among his 11 Grand Slam titles. Nadal has also won six French Opens in his 10 major titles.