Williams, who looks in fine form for the defence of her French Open title which begins next weekend, overpowered her Romanian opponent 6-4 6-1.
The world number one will now be a hot favourite to land the 70th career title of her storied career on Sunday when she faces the 24th-ranked Keys, who defeated Spain's Garbine Muguruza 7-6(5) 6-4 in the other semi-final.
Williams has looked vulnerable this season, has not competed for two months and pulled out of last week's tournament in Madrid with flu yet she demonstrated that she will still take some beating at Roland Garros despite playing only one claycourt tournament.
Williams wins first title in nine months in Rome
There were brief interruptions for rain on a dank, wet day but after Begu, ranked 35, had extended Williams in the opening set, the 34-year-old dismissed her challenge mercilessly in the second.
Keys has been enjoying her best claycourt performance in Europe and caused a shock with her win over Wimbledon finalist Muguruza but beating Williams, who has won 15 successive matches against Americans, is a challenge of a very different magnitude.
Earlier, Andy Murray raced into his second major claycourt final in a week with a comprehensive 6-2 6-1 win over outclassed Frenchman Luca Pouille in the men's event.
The Scot, who lost to Novak Djokovic in the Madrid final last weekend, continued his fine form by keeping his concentration through two rain delays and taking a minute under one hour to dispose of qualifier Pouille.
The 22-year-old Frenchman has an exciting future but, at number 52 in the world, the 49-place gap with twice Grand Slam champion Murray was painfully evident in a one-sided contest.
The number two seed has yet to drop a set this week and is poised for a second straight final meeting with world number one Djokovic, who faces Kei Nishikori in his semi later on Saturday.
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