Williams is fifth on the Grand Slam singles title count with 17. Australian Margaret Court leads the list with 24 titles, but many of those were won before the Open Era began. Steffi Graf, who retired in 1999, is second with 22 Grand Slam singles titles; Helen Wills Moody is third with 19. Martina Navratilova and Chris Evert own 18 major titles apiece.
"Serena is the best ever to me," Azarenka told AFP. "I cannot judge somebody who I never played before but for me overall the game of tennis has elevated and the evolution of the game raised up so much that it's hard to compare.
"It’s like comparing Muhammad Ali and Floyd Mayweather. How can you compare two eras? They're both the greatest in the world, but who was the better boxer?"
Azarenka also said she is closing the gap on Williams, even though she has yet to beat her at a major. The world No. 2 handed Williams two of her four losses this season in the 2013 Doha and Cincinnati finals, but lost the U.S. Open final to her in three sets.
"Serena is playing the best tennis of her life and so am I," Azarenka said. "It's been really noticeable the gap from last year we have in the matches has minimized. I've beaten her twice this year. No one else has done that. It's just exciting to be in the era of somebody and competing with somebody who is considered the best ever and being their toughest opponent."