It was quite clear that the athletecan be able toimprove upon his own record.

The underlined part ‘can be able to’ must be replaced with ‘would be able to’ to match the tense in the sentence.
Other similar examples:
Ex. He came at 8: 30 pm, freshened up and sat (not sit) by the window with his favorite novel.
Ex. It was obvious that he smoked (not smoke) in nobody’s presence.
Ex. He said he would (not will) go to Lucknow on Friday.