The last Premier final before Roland Garros will feature two Americans at different stages of their career and with contrasting clay court pedigree - here are 10 Things To Know about the Internazionali BNL d'Italia final:
1) Williams has a fine record against fellow Americans.
Since losing to her sister Venus at Montréal in 2014, Williams has won 15 straight matches against Americans, including two against Keys (at the 2015 Australian and US Opens). Her last loss to an American in a final came way back in 2008, against Venus at Wimbledon.
2) The Eternal wait is over.
The Foro Italico will host the first all-American final on tour in nearly four years (Serena d. Vandeweghe, 2012 Stanford), and the first in Rome since Billie Jean King defeated Julie Heldman in 1970.
3) Williams in unfamiliar territory.
For the first time since 1998, Williams is contesting this tournament without already having a won a title in the season to date. Keys has also endured a slightly below par start, failing to make it beyond the quarterfinals of an event until this week.
4) Keys could secure a French Open boost.
Keys will move back into the Top 20 by virtue of reaching the final. Should the American lift the title, she will guarantee herself a Top 16 seeding at Roland Garros, avoiding the big guns until at least the fourth round.
5) Good omen for Keys.
The last time Keys defeated two Top 10 players in the same week, she won the title (2014 Eastbourne - No.7 Jelena Jankovic, No.9 Angelique Kerber). In Rome she has already overcome No.9 Kvitova and No.4 Garbiñe Muguruza. Will lightning strike twice?
6) Williams has won her last 19 matches in Rome.
Williams loves the Foro Italico clay, winning her last 19 matches there. Her last loss* at the Internazionali BNL d'Italia came in a third set tie-break against Jankovic in 2010.
* Injury forced Williams to withdraw prior to her matches with Christina McHale in 2015 (elbow) and Li Na in 2012 (back)
7) Williams is trying to win her fourth Rome title - and 70th across all tournaments.
On Sunday afternoon, Williams hopes to lift the 70th title of her illustrious career, and the fourth in Rome. The other events she has won four or more times are the Australian Open (six), Wimbledon (six), US Open (six), WTA Finals (five) and Miami (eight). The only players to be crowned champion on four occasions in the Italian capital are Chris Evert (five), Gabriela Sabatini (four) and Conchita Martinez (four).
8) Keys is going for the biggest title of her career.
Perhaps surprisingly, Keys' only WTA title to date came two years ago in Eastbourne. Despite her self-confessed preference for the tour's faster surfaces, her only other final did come on the green clay of Charleston, ending in a three-set loss to Kerber last April.
9) Every underdog has its day.
World No.24 Keys is not the only unseeded player to make it to the final in Rome. In fact, in 2010, No.26 María José Martínez Sánchez managed the feat, defeating Jelena Jankovic to lift the title.
10) Williams is going for her 13th clay court title, the most of any active player.
Williams has the most WTA clay court titles among active players with 12. She still has quite some way to go to catch the all-time leader, Chris Evert, who lifted 66 trophies on the surface.