That being said, I see Novak Djokovic winning this match. Nadal is bidding to clinch the year-end number one spot in the ATP rankings for a fifth time - he is now 640 points ahead of Djokovic ahead of the season-ending tournament in London, starting tomorrow.
'I don't want to lie to you, he said.
Although the second-ranked Serb, who won four straight titles at the year-end event in London between 2012-15, only needed to hit a total of 10 winners in the match as Berrettini's 28 unforced errors handed him an easy victory to start round-robin play.
Roger Federer says he is playing knockout tennis in his remaining two ATP Finals matches after falling to a surprise 7-5, 7-5 defeat against Dominic Thiem.
"I thought also Dominic saved himself real well when he had to, so I thought he played a tough first-round performance today".
"But I really hope that I will be able to serve every single day a little better, and my goal is to be on Sunday serving normal". I'm not feeling uncomfortable because I haven't really served. "I'm very happy that I pulled through the last game".
"Had my chances, I felt, you know".
Nadal, who won the French Open and US Open this year, said he had been happy with his form before his injury. The 19-time Grand Slam victor said, "I am sure that I will compete well, but this tournament is what you play with the top players, so you have to be 100 percent prepared".
The Swiss star will take on Matteo Berrettini on Tuesday and Novak Djokovic on Thursday.
World number two Djokovic can overtake Nadal to become year-end number one this week in London but must reach the final to stand a chance.
The 32-year-old needs to reach the final of the season-ending showpiece to reclaim the number one spot from Nadal and match Pete Sampras's incredible record of finishing six separate seasons as the world's number one player.
Nadal, who has fitness concerns due to an abdominal problem, faces defending champion Alexander Zverev on Monday evening. In order for me to be in the position to be year-end number one I understand I have to be healthy and schedule well.
But all eyes are on a mouth-watering repeat of the Wimbledon final in Group Bjorn Borg later in the week - the top two players from the four-man group qualify for the semi-finals.