"The last few days I start to feel a little bit the abdominal, especially when I was serving", Nadal said.
"It has been a tough year for me in terms of injuries so I want to avoid drastic things", Nadal said.
Rafael Nadal and Novak Djokovic, both playing in Paris Masters this week and fighting for the World No.1 ranking, say they are reevaluation their scheduled exhibition match in Saudi Arabia following condemnation of the country's government.
The 31-year-old Serb has returned to his brutal best this season with Grand Slam wins at Wimbledon and the U.S Open, along with ATP Masters triumphs in Cincinnati and Shanghai.
Federer enjoyed a walkover into the third round when Milos Raonic pulled out with an injury.
The 34th-ranked Krajinovic served for the first set but rapidly came unstuck and suffered a 7-5, 6-2 loss that will see him tumble outside the top 90.
"I am not thinking about big improvements or big things".
The fifth seed bounced back from a bruising defeat to Anett Kontaveit on Wednesday, but overcame Mertens 6-2, 7-6 in a tense round-robin tie in Zhuhai, southern China. He went on to trail 4-5 while leading 40-15, but let slip his advantage in the tenth game to lose 6-4 in the first set.
Dimitrov has been coached in Paris this week by Andre Agassi, a former coach of Djokovic.
Kei Nishikori and John Isner kept their hopes of gatecrashing the Tour Finals alive with victories in Paris.
Initial whistles from the Parisian crowd at the delay were eventually replaced by clapping as the Serbian ace retrieved a towel to hand to the spectator, who was later helped from his seat by medical staff.
Khachanov booked a quarterfinal encounter with No 4 seed Alexander Zverev, who got past Diego Schwartzman 6-4, 6-2.
Djokovic can regain top spot from Rafael Nadal this week if he betters the Spaniard's performance in the French capital, where the Serb is a four-time victor of the indoor tournament.