Alastair Cook reached an elusive and rewarding milestone with his 20th Test hundred on day one of England's Investec Test series against South Africa.

The England opener, who put on 170 with Jonathan Trott (71) after Andrew Strauss made a duck in the first over at the Kia Oval, had gone 16 innings since turning his 19th century into a career-best 294 at Edgbaston last August.

His unbeaten 114 not only served the world number ones well in their 267 for three, as they defend their status against their third-placed visitors, but took Cook level with his Essex mentor and England batting coach Graham Gooch as well as team-mate Kevin Pietersen in his country's list of most prolific centurions.

He made no mistake when the time came, pushing an undemonstrative single to cover off leg-spinner Imran Tahir to complete his five-hour hundred in 222 balls - having hit 11 fours and one six.

England's day could hardly have started any worse when Strauss was lbw to Morne Morkel, via DRS. It took some courage too, as well as good judgment, to risk a review so early in proceedings - after umpire Steve Davis had turned down Morkel's lbw appeal against the left-hander from round the wicket.

Trott drove his first ball calmly past mid-on for four - and a frantic first over concluded with a Dale Steyn misfield in the same position, and two more runs.

Captain Graeme Smith held Steyn back for almost an hour, in awkward batting conditions under floodlights. Cook and Trott stayed patient, but when South Africa dropped short they began to pick up boundaries across the never-ending Oval square.

For good measure, Cook also counted six with mis-hook at Steyn into the stand at long-leg - only the sixth six of his Test career. When Cook passed his 50 in mid-afternoon, he also brought up his and Trott's seventh century stand together.

Comparisons with their unbroken 339 in the famous draw in Brisbane at the start of England's 2010/11 Ashes series victory were perhaps a little premature, and in the end the world and Test match players of the year made it only just past halfway to that number before Trott edged a drive behind to give Morkel his second wicket.

His typically determined innings had nonetheless lasted 162 balls, and set the stage not just for Cook to continue but Pietersen to up the ante with his range of stroke. The latter was caught behind, aiming a pull at Jacques Kallis who ended a stand of 81 with the old ball, but it was still a chastening day for South Africa.