India, Sept. 10 -- Karen Khachanov and Frances Tiafoe, the two unlikely US Open semi-finalists by pre-tournament seedings, have plenty of parallels. Both played their first full Grand Slam season in 2017. Both are into their third Major semi-final. Both are yet to make a final.

Another striking resemblance followed their contrasting quarter-final victories. After Alexander Blockx retired at 6-2, 7-5, 3-2 down on Wednesday, Khachanov engaged in a long chat with Blockx near the net before starting his own muted celebrations. Even there, he asked the crowd to give it up for his injured opponent. A day earlier, after Tiafoe pulled off a five-set heist on match tiebreak from two sets and a break down, he rushed to the net as Alex Michelsen bu...