Strong earnings from Nike and Tiffany boosted stocks as investors saw a chance to add to their holdings after declines earlier in the week. Nike reported a surge in quarterly profit, while luxury jeweler Tiffany topped predictions, helped by demand from Asia. A damper, however, on US markets came from Cyprus, a banking haven, as it struggles to devise a plan to avoid financial collapse. Despite concerns, a resilient global economy has encouraged investors to pick up stocks on any dips, said a managing director of investment strategy at Wells Fargo Private Bank. ‘‘We still have an astonishing amount of money sitting on the sidelines,’’ he said.