For Simona Halep, the distinction is clearcut.
As the reigning world No.1, the Romanian is one of 15 female tennis players officially ranked above history-chasing Serena Williams.
But Williams' career tally of 23 majors - just one behind the all-time record held by Australian Margaret Court - dwarfs the combined total of nine currently owned by the top 15 on the world list.
Halep has sat atop the rankings for much of the past two years and had her status validated when she broke through for her major grand slam crown at Roland Garros last year.
"In my opinion, to be No.1 in the world and to be the best player in the world, it's a little bit different," Halep said ahead of Monday's heavyweight fourth-round showdown against Williams at the Australian Open.
"In this moment, I'm No.1 in the world, so I will take that.
"I feel like I have been there many months, many weeks.
"But for sure she's the best player in the world because she's won so many grand slams.
"She's been a lot at No.1 so I cannot compare my results to her."
Williams has also dominated their career head-to-head record, with Halep's only victory in nine attempts coming at the 2014 WTA Finals in Singapore.
But that doesn't mean Halep doesn't fancy her chances of beating Williams at Melbourne Park.
Nor is she intimidated by the American. At least, not any more.
"I have huge respect for her because she's a great champion, but she's just an opponent next round," said Halep.
If Williams needs any first-hand advice on just how well Halep is travelling at the 2019 Open, she need go no further than older sister Venus, who bowed out 6-2 6-3 to the Romanian in the fourth round on Saturday.
"I guess I'll talk to her," said Venus.
"I don't know if Serena needs my help or not.
"If she does, I'll be there."
The winner will take on either No.7 seed Karolina Pliskova or two-time major champ Garbine Muguruza in the semi-finals on Wednesday.
Australian Associated Press
