For Saudi Arabia, Sisi's win sealed the end, for now, of the rise of the Brotherhood, the international standard-bearer of mainstream Sunni political Islam since it was founded in Egypt in 1928.
Since the movement has a following in every Arab and Muslim society, the example set by the Egyptian Brotherhood's embrace of the ballot box poses a threat to the hereditary dynasties that hold sway in wealthy Gulf Arab states such as Saudi Arabia and the United Arab Emirates.
The movement enjoys substantial support from Islamist-friendly countries like Qatar and Turkey and its influence appears to have grown in countries such as Morocco, Tunisia and Libya since 2011.
The blunt, emotional language in the king's message - which described troublemakers in the region as "the helpers of Satan and its troops on the ground" - laid bare an underlying power struggle in the region pitting conservative, anti-Islamist governments, represented by Saudi Arabia, the UAE and Egypt against countries such as Qatar and Turkey who appear to believe that Brotherhood represents the future of Arab politics.
Sisi, like Saudi Arabia, is no admirer of the Shi'ite Muslim clerical rulers of Iran, whose administration is seen by Riyadh as a expansionist power bent on exporting its Islamic revolution to the Arab world and interferings in the affairs if neighboring Gulf Arab states. Iran denies any such interference.
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Senior Saudi and Gulf officials are expected to turn out in force at Sisi's elaborate inauguration to show the Egyptian army filed marshall has the backing of a new regional order that applauded his bloody removal of a Brotherhood-led government in 2013.
Qatar, which poured billions into Egypt during Mursi's one year in power, was not invited. And in a sign that Western allies are not entirely comfortable with the state of Egyptian democracy since then, they only plan to send low-level representatives to the event.
Saudi Arabia, the world's top oil exporter, was livid when autocratic Egyptian ruler Hosni Mubarak was toppled following popular demonstrations in 2011 which eventually led to the Muslim Brotherhood, long mistrusted by Riyadh, coming to office.
Riyadh and the UAE have joined Sisi in clamping down hard on the Brotherhood, seen by them as 'terrorists', in an effort to quell regional instability wrought by the Arab Spring.
Their overriding goal is to prevent the Islamist movement leading the Arab world's most populous country once more.
"Ugliness of evil"
The message seemed meticulously orchestrated.
Less than 15 minutes after Sisi was officially declared Egypt's president on Tuesday, Saudi King Abdullah issued his ringing statement of support. He also called for a donor conference to help the new president fend off economic collapse.
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The statement went beyond the usual, terse messages of congratulation. From his holiday home in Morocco, the king went a step further: he said the "infringement" in Egypt's affairs would be considered an infringement of Saudi's affairs.
In a thinly-veiled reference to the Brotherhood, and perhaps also to Qatar, the king said: "I warn you all against the ugliness of evil, for it has a very dark face and only works for its personal interests."
Saudi Arabia regards the Brotherhood, the oldest Islamist movement in Egypt, as an existential threat since their embrace of elections challenges the Gulf tradition of dynastic rule.
The king, who has long been infuriated by the support shown to the Brotherhood by Qatar, demanded non-interference in Egypt's affairs.
"It is a non-bargainable and non-negotiatable position under any circumstances," the royal statement read.
In an unusually public spat, Riyadh punished Qatar for its pro-Brotherhood stance by taking the unprecedented step of recalling its ambassador from Doha earlier this year.
The message also signaled that Riyadh, long seen as the 'big brother' in the Gulf, expects Egypt's allies to step in and provide Sisi with the resources he needs to rescue an economy damaged by three years of tumultuous politics.
Cairo's European allies, however, appear to be less amenable to the message.
The EU said on Thursday it was concerned with the detention of political opponents, activists and journalists. A Western diplomatic source said there had therefore been a "collective decision" to send only ambassadors to the inauguration.
"No place among us"
The king said that anyone who did not use their resources to help Sisi "would have no place among us tomorrow, if ever they were to suffer from ordeals and crises".
Saudi Arabia could barely contain its glee when Sisi toppled Mursi, a veteran Brotherhood Islamist, after Egyptians demonstrated against his rule.
It has since pumped