Carlos Alcaraz said he has no doubt that top-ranked Jannik Sinner is the best player in the world on current form as the two young players continue to lay the foundations for an epic rivalry.
Alcaraz, the 21-year-old world No 3, won the French Open and Wimbledon titles last year, splitting the four Grand Slam tournaments with 23-year-old Sinner.
The Spaniard also improved his head-to-head record with the Italian to 6-4, beating Sinner three times in as many meetings.
Sinner, meanwhile, won 73 of his 79 matches last year including the US Open and the Australian Open, which he successfully defended at Melbourne Park last month.
"Jannik is the best right now," Alcaraz told reporters in Rotterdam on Sunday.
"He has lost only four or five matches in the past year, so it's crazy. I know people say who's better out of us. They say Jannik is better or some say me. It's all discussion.
"But for me, I think for a tennis player, we have to face Jannik and he's winning everything. He's focused every time, so I think he's the best. Every tournament he plays, he makes the final or lifts the trophy."
Alcaraz returns to action in Rotterdam after his quarterfinal loss to 24-time Grand Slam champion Novak Djokovic at the Australian Open ended his bid to become the youngest player to capture all four major titles.
"I don't feel that it was a missed opportunity against Novak," added Alcaraz, who won the US Open in 2022.
"I really wanted to win the tournament and felt I was able to, but Novak played an unbelievable match. Facing Novak in the quarterfinals at a Grand Slam is the worst thing.
"It was a pretty good match. I've tried to take the good things about that match and will look ahead now."
Top seed Alcaraz begins his Rotterdam campaign against Dutch wild card Botic van de Zandschulp, who beat him in the second round at the US Open in August.