Alastair Cook believes there are still "a lot of unanswered questions" over the England batting line-up and said "places are up for grabs" following the Test series victory in South Africa.
Alex Hales was unable to solve the conundrum over who should open with Cook, while Nick Compton and James Taylor both struggled for runs after decent performances in the first Test as England clinched a 2-1 series win over the Proteas.
Kagiso Rabada took 13-144 - the second-best match figures in history by a South Africa bowler - as England crumbled to 101 all out on the final day of the fourth Test at Centurion to suffer a 280-run defeat.
England skipper Cook knows there are problems to be addressed ahead of the next Test series against Sri Lanka - which starts in May - despite an impressive series victory.
Cook said: "There are still a lot of unanswered questions in our top seven batting. There are certainly places up for grabs.
"The output we've had in this series hasn't been good enough if we're trying to get to number one in the world, which is the ultimate aim.
"As a batting unit we've relied too heavily on Joe Root, Ben Stokes and Jonny Bairstow. Absolutely we've got room for improvement.
"At the end of the day, results matter and your end column of runs are absolutely vital. So to say they've totally convinced me would be wrong."
Cook said the pressure will be on the batsmen when the County Championship season gets underway in April.
He added: "Those guys have got to continue working hard and make sure that, when selection comes round for Sri Lanka, they have scored a bucket load of runs for their counties."