Shanghai (AFP) - Double 2015 major winner Jordan Spieth is officially back on top of the world after leapfrogging Australia's Jason Day in the new global golf rankings released on Tuesday.

Spieth's tied seventh place in the WGC-HSBC Champions in Shanghai on Sunday was enough to take the Masters, US Open and FedExCup champion back to world number one as Day opted not to play to be at the birth of his second child.

Rory McIlroy started the year as number one but lost that position to Spieth after the US Open in June and dropped to third after missing the defence of his British Open crown when he injured an ankle playing football.

Day has won six tournaments in 2015, including the PGA Championship in August during a run of four titles in six starts as he reached number one for the first time.