Keitany won in 2011, 2012 and 2017 - when she set a women-only record (without male pacemakers) of two hours, 17 minutes, one second.
Last year, she set her sights even higher when she ran with male pacemakers in a bid to break Paul Radcliffe's world record of 2:15.25 - but paid for her early efforts and was overhauled by fellow Kenyan Vivian Cheruiyot, finally fading to finish fifth.
The two are likely to set the pace again on Sunday but it is a tough race to call; race director Hugh Brasher said it was "undoubtedly the best women's field ever put together for a major marathon".
Kenyans Gladys Cherono, who has won Berlin three times, and Brigid Kosgei, last year's Chicago winner, have both run under 2:19 while a group of three Ethiopians - Roza Dereje (2018 Dubai champion), Birhane Dibaba (2018 Tokyo champion) and the promising Haftamnesh Tesfay, who turns 25 on race day, are only marginally behind them in times.
Cheruiyot switched to marathons only in 2017 at the age of 33 after a track career that brought Olympic and world titles over 5,000 and 10,000 metres.
While she judged her race to perfection to reel in Keitany in London 12 months ago, her rival later gained revenge in New York. Cheruiyot then provided a reminder of her track speed by scorching to a 66.34 half-marathon victory in Lisbon in March.
"I did a personal best in Lisbon and what I can say is that I am in better shape than last year," she told journalists on Thursday.
"I'm learning a lot in the marathon now. When I did my first one in 2017, I was like, 'Am I going to finish it?'. But now I’m getting used to it. This is my fifth marathon and I’m really enjoying the training."
Keitany has taken heart from her 2:22.48 performance on the tough New York course, where she finished over a minute ahead of Cheruiyot.
"I ran fairly fast in New York on what is not an easy course so I wanted to get enough recovery - which is why I cancelled a half-marathon so I could prepare for London well," she said. "Everything has been going well in training, so I'm ready."
Keitany dismissed the idea that it was a two-horse race. "Everybody here is in good form, that’s why we are here," she said.
(Reporting by Mitch Phillips, editing by)
