UA 3873 then UA 4521 at O'Hare
Fort Worth → Chicago → Greenville-Spartanburg
Timetable, Chicago time
17:55UA 3873 from Fort Worth at the gate
06:29UA 4521 to Greenville-Spartanburg leaves the gate
12 h 34 apart on the timetable, overnight
UA 4521 leaves 12 h 34 after UA 3873 arrives on the timetable: more than half a day, so not a connection we count.